Buying used means no warranty, but it also means you will pay much less than when they were new. Bikes seem to suffer a great amount of deprecation the first couple of years. On a used carbon bike, it is a lot more risky because of the no warranty. My first two bikes were used, so it is not as big a deal as some might think. If in doubt have an LBS inspect any possible bike you will purchase.

As for actual guidelines are not sure what's out there.