Great advice thanks..Big help is knowing what model year you are buying. You can get some big bargains where someone has bought a bike new, ridden it a handful of times, realised cycling requires more effort than driving a car and then decided to sell it.
I don't know about other brands, but Specialized's also have a build date decal on them (presuming the original owner hasn't removed it).
If you are buying a bike thats just a few months old getting it serviced after you have bought it will save you sometimes hundreds of pounds over buying a new one........