When you buy a computer on eBay you must really understand what is or is not included before you bid, as sometimes parts are not mentioned in the description or described in a way that is easy to understand and compare.
This guide should help point out some of the more important items to be aware of. If you find it useful please vote for it.
Here are some of the factors to consider in order to meet your needs at the price you are willing to pay:
The Processor or MicroprocessorThere are basically two competing manufacturers of this component, Intel and AMD, and their products are not interchangeable although they perform similar functions. These processors are very confusing in that they come in many types and speeds, some of which are far more powerful than others yet sound similar in name and are very different in cost. (This is true of laptops too). This is where you can easily think you are getting a bargain when you are actually buying a far inferior computer. The current trend is to have two, three, four or more Processors on one 'chip' and these are dramatically faster than single Processor based PC's for many uses. However the very latest and fastest Processors cost much more money than ones nearly as fast. If you don’t really need the thrill of temporarily having the most recent Processor save a lot of money and buy the next one down in power in the same family. Please avoid Celeron Processors from Intel, as they are not very powerful, and not much less in price than the more powerful ones from either company.
The Mainboard (or Motherboard)These can only support certain features and certain capabilities. They determine which family of Processors will work in your computer, how much memory you can have in total, and how many expansion slots there are. If you ever expect to upgrade later, ensure that you have enough space in all these ways for future needs. New boards come out every month and are vastly different in capabilities. One thing to be aware of is that some boards need memory to be added in pairs. This means you will often need to buy more memory than the software can actually use. If you are unsure what you need - ask a competent friend to advise you.
The Memory (RAM)The type, speed and quantity of memory in your PC will make a huge difference to the performance, but surprisingly it makes less difference to the cost. More memory, measured in Gigabytes or GB, is better until you reach the limit that your Operating system can use. Not enough memory will mean everything runs slower – even internet access. The absolute minimum is 1GB and if you can afford more – get it but note that your Operating System will only be able to use a maximum amount of memory, currently only 3GB for Windows XP 32Bit or Windows Vista 32Bit versions, which are the the most common in use in the home BY FAR. So more memory than this is a waste except for Motherboards that need memory in pairs as described earlier. There you need to buy 4GB in order for Windows to use 3GB of it. Every type of memory has a speed rating and these ratings are critically important measures of performance. Some memory can even be run above its rated speed (called over-clocking) but leave this to the experts that design the system or your knowledgeable friend to do. The combination of memory and motherboard is important as it must be matched carefully.
The Video CardNewer, faster and cheaper video cards are coming out almost weekly. The cost differences are enormous between very similar sounding cards, and here is one way the Seller can make a profit by knowing more about the video card that is included than you do. You only need to buy the included video card that can handle the type of programs you will commonly run. Gamers need very powerful cards, and there can even be multiple video cards in some truly expensive systems. Most users do not need expensive video capabilities. There is a variable amount of memory on a video card too, and each card has a particular speed that it runs at.
Many eBay negative feedback comments I have seen are left for computer Sellers because of a misunderstanding about the video card. A common ‘trick’ is for the Seller to refer to the video card (using the Manufacturer own overly-technical words) in a way that implies that the card has a lot of memory on it – when in fact the card shares the memory installed elsewhere in the computer. These video cards will be much slower (and dramatically cheaper) than fast cards with lots of 'latest generation' memory installed on them. Even worse there are whole confusing families of cards and usually within families a larger part number means a more recent and more powerful card – but not always. If you are unsure what you need - ask a competent friend to advise you.
The Operating SystemWhile Microsoft Windows dominates, there are currently multiple Windows versions available, and these do make a difference to your experience of using your computer. They also have slightly different prices and capabilities. The safest way to cover this point is by referring you to Microsoft’s website where you can see the Operating Systems compared. Choose carefully. The Operating System will not care which Processor you have, but the more recent Operating Systems need more memory and can take better advantage of the latest Processor features. There are good non-Microsoft choices too, such as Linux, which is nearly free. As with most of these topics, if you are unsure what you need - ask a competent friend to advise you.
StorageNowhere has progress been more astonishing than in the improvement in size, speed and reduced cost of hard disks (also called hard drives). Every computer needs at least one, and the price difference between them is relatively small. These drives are also measured in Gigabytes (GB). Usually the more GB the better, as you will store all your programs, music, pictures, and video and data files here. Another thing to look out for is the drive rotation speed in RPM (faster usually equals better). There are different drive standards and the more recent one is called SATA II. It is beyond the scope of this guide to go into the differences but a SATA II drive of at least 250 GB or bigger that spins at 7200 RPM or faster is a great choice for almost everyone. Bigger and faster is better here, but some drives also have very good self-protection capabilities. This is important as most people do not carefully back-up (keep a proper copy of) their computer’s data and a fault in hard disk can lose everything you haven’t copied elsewhere.
Also installed will almost certainly be some type of disk drive. Blu-Ray is the latest standard (or its competitor HD-DVD), but these are still very expensive, so most computers are being sold with DVD-Writer drives. DVD-Writers will play and record CDs and DVDs, and at least one drive is essential on every computer. These also come in different speeds and capabilities but the drives are very inexpensive so make sure you get a fast one.
The Power SupplyThere have been very significant improvements in power supplies recently and this is partly to accommodate the huge power demands of the new Processors and video cards. The more powerful and quieter the power supply is the better. Please note that these do not add much cost to your computer, and do make a big difference in terms of noise, expansion, and reliability of the computer.
Buy yourself at least a decent separate surge suppressor (here price=quality), but also consider an uninterruptible power supply (UPS). Either of these will save you from most power problems but only the UPS will also keep you computer running if there is a (common enough) loss of mains power. This gives you time to save your work. Your computer will certainly last longer with power protection installed.
The CaseThere is much more to a case than appearance. There are cases which keep the computer extra cool with fans or other cooling devices or the case material itself, which helps it run better and last longer. Different cases can accommodate more or less storage, and have convenience features like ports in the front or tool-less assembly for ease of internal access. After these functional features the rest is cosmetic - including shape, colour, lights, and displays. Cases are not very expensive. And you cannot go too far wrong no matter which you choose.
Ports and ConnectionsYou need USB ports. The current standard is USB 2.0 and the new USB 3.0 standard should show up soon. Do not buy a computer that is only USB 1.0 or USB 1.1, as it is out-of-date. This is another way Sellers can get a little more profit as USB 1.1 or 1.0 items are worthless and should not show up on any new computer except as a 'legacy' supported feature. Windows Vista is very particular about the USB devices it supports. Try to only buy USB devices (webcams especially) that are Vista Certified as you will be wasting time and money on anything else if you move to Vista later on.
The video card will have at least one port to connect to a monitor. This port can be Digital or Analogue (older). The important thing is that it is compatible with the monitor you intend to use. The better monitors and cards have the Digital port and I recommend this unless there is a cost-saving reason for not getting Digital. There are inexpensive adaptors to convert Digital to Analogue if you have an older or cheaper monitor already. These often come with the video card.
Firewire (also called IEEE 1394 or i.Link) is another standard. It is faster than USB 2.0 but much slower than USB 3.0. Personally I say stick to USB but firewire ports are a bonus in case you have to connect something that has one - usually a video camera.
If you have any intention of connecting your computer to another computer or a network, make sure you also have an Ethernet port. Although there are various speed of these ports the cost is so miniscule now that one is probably built into every computer anyway.
Finally, a reminder that if you found this Guide useful please vote for it.
For more precise descriptions and more detailed explanations of any of these topics, have a look at Wikipedia the free internet encyclopaedia.