It has now been several years since small, portable video projectors came into the marketplace, and a number are becoming available on the second hand market. However, there are a number of pitfalls of which potential buyers need to be aware.
Firstly, prices of new projectors have dropped by around 50 - 60 % in the last 3 years, so look at new prices before buying used - you may be pleasantly surprised, and remember, you get a warranty with a new machine, now usually 3 years - you won't get this on a second-hand device. Also be aware that this is an area of technology that is changing very quickly with continuous improvements in reliability, running costs, size and noise.
Secondly, replacement lamps for older machines can be very expensive, sometimes as much as £400, and there is no way of telling how long a lamp in any projector will last - although lamp life may be quoted as 2000 hours, lamps often fail in less than half this time, particularly if the projector is moved around a lot. Lamp life can also be seriously affected by use in cold, hot, humid, smoky and, particularly, dusty conditions. Always assume you will have to fit a new lamp sooner rather than later, and find out how much a new lamp costs before committing to a purchase. Recent projector models tend to use smaller, cheaper lamp units than older models - for example, a 2000 lumen projector from 3 years ago would probably have used a 250W lamp, with a replacement cost of around £350; whereas a new projector with the same light output will use a 200W lamp, costing under £200 - so research lamp costs carefully. You cannot rely on any internal lamp-hours run counter as they can be easily reset by an unscrupulous seller. If you buy a projector that isn't working, fitting a replacement lamp may not resolve the problem - other internal components such as the ballast can fail, or the projector may have been dropped and have a damaged circuit board - then you are looking at expert repair charges on top of the cost of a lamp. If you can't see it working, avoid, even if it is cheap.
Thirdly, check where the projector came from and how it was used. Projectors are very popular in pubs and nightclubs, where they often get a hard life, particularly from smoke, humidity and sonic vibration. DLP projectors are a better bet than LCD devices as the optics are better sealed against smoke damage, but used projectors from these sources are rarely worth having unless they have a guarantee included. The same caveat applies to ex-rental items. Unfortunately, projectors are also prime candidates for theft, so always ask to see the seller's original purchase receipt.
Fourthly, getting any servicing or repair work done out of warranty is difficult and expensive - in most cases, broken machines will need to be shipped back to the manufacturer as they are the only people who have access to the necessary spares and expertise - and they will probably quote a standard repair cost of several hundred pounds; however, they will usually give a limited warranty of 3 months on any repairs. These are not items that can have faulty components easily repaired by your local TV repair company - they all feature multi-layer PCB's with through-plated holes and densely-packed SMT components, and need a purpose-built fault-finding diagnostic rig to track down faults - even the manufacturers don't usually bother with this, they just swap in a new circuit board and throw the old one away.
The bottom line of all the above is that used projectors only have any significant residual value if they are still under warranty, which is why rental companies sell off their stock after 6 months. We have all become used to electronic items remaining reliable for long periods of time - however, this does not apply to video projectors. After a projector's warranty has expired, it should be regarded as being 'on borrowed time' - any fault repair other than a lamp replacement is unlikely to be economically viable, especially given the low cost of new projectors, so think carefully before parting with your hard earned cash for any used machine that doesn't have a warranty - just because a projector cost £1500 3 years ago doesn't mean that it is worth anything like that now, or indeed, anything at all - as IT equipment, any business will have written it off 100% after 3 years.
Finally, if you are looking for a projector for home cinema use, you don't need an XGA (1024 x 768) resolution imaging chip - an SVGA (800 x 600) unit will work fine with video and will cost less; however, do look out for adequate brightness - around 2000 lumens - and component video input for the best picture quality, and try to pick a model which has a low fan noise - once again, newer models are often significantly quieter than their older counterparts.
** Additional information **
Time marches on, and technology with it - since I originally wrote this article, new video projectors have become even cheaper, to the extent that it's often more economical to buy a new projector than a replacement lamp; however, a new lamp technology is emerging which looks like it will make all projectors that use conventional - i.e. short-arc halogen - lamps obsolete. This new technology, being pioneered by Casio, for example with its XJ-A 240 projector, uses a light source that combines LED and laser sources, has a 20,000 hour lifespan and produces very little heat, so no need to let the projector cool before moving it. As the XJ-A 240 offers 1280 x 800 resolution and 2500 lumens brightness, all for under £1000, the writing is on the wall for older devices.