Buying a second hand PC from ebay can really save you money, but there are some negs to take in account, in most cases you only have a short warranty or none at all, Also a big thing is the posting. A computer is heavy and would have to be packed well, so the posting cost is going to be high, So if you are selling a PC get its weight checked out at the post office before putting on ebay and remember to add the cost of packing, because it will have to be packed well.The other thing to take in account is that the PC no matter how well packed is going to be chucked around in a back of a delivery van, which does not do the PC or hard drive any good.So i would say the best thing to do is to look for a computer with out a hard drive, after all when i have bought a second hand computer the hard drive is the first thing that i change and hard drives have gone right down in price OR if you can pick up the computer yourself and ask to see it working.
When looking to buy a computer remember to buy only what you need one for and what you can afford. If you want a family computer with internet and play the odd game on, i would say a 1700-2000 processor with 80 gig hard drive and half a gig of ram would be fine. Remember you can get a lot fastest PC than this and alot bigger hard drives, but do you really need it apart from keeping up with the neighbours. I know some people will say you need the extra memory, ram, hard drive space to play bigger and better games, in that case buy a games console.
Another good idea is to buy the bits and build your own PC. You learn more and it can be great fun.When buying motherboards and processors and power units i would say only buy new.That way they is less chance of things going wrong, more so if you are a first time builder.
When selling a computer remember to wipe all your personal information from the hard drive or just sell it with out a hard drive, it is the safest way. The last thing that you want is your personal information to get into the wrong hands.
You might be thinking of buying a computer from a shop, but remember if you do and you want to upgrade the PC once you take the case off, you will not have a warranty anymore.Shops also offer you extra warranty at a extra cost and i have found that they are just not worth it.To much small print and they always find a get out claws.
Thanks for taking the time to read this, I hope some of it has be usefull.