This is where a good,trustworthy pro shop can come into play. I would never purchase a used bow (or new for that matter) without inspecting it and shooting it numerous times first. Give the limbs, cams, and risera very close inspection looking for any cracks, ask your pro shop if they will throw it on a press and ask their opinion, try to buy from a source that you trust as much as possible,find outif there are any know problems with that bow model, and lastly shoot it some more, listen closesly for any cracks, squeaks, etc. Bows are unlike guns and will deteriorate over time even if well cared for.

Oh, and buy a Hoyt cause they are tough