Your best bet is to check for some deals in the Buy/Sell forum on this site or on FredMiranda.com. I just bought a used 5D in mint condition with very low actuations from the FM site (matter of fact, it got delivered about an hour ago). If you do by used, you'd generally be better off buying from an established member of the particular site (look for number of posts, member since, etc). Ask a LOT of questions (you just need to be careful):
1.) why are you selling?
2.) are you the original owner?
3.) how long have you owned the item?
4.) can I return it for a refund if I'm not satisfied?
5.) pictures of the item?
If you're concerned about warranty, you'd be better off going to a reputable dealer (as someone else has suggested). I've done a lot of business with B&H and Buydig.com over the years and never had a problem. If you buy from a dealer other than these two, be sure to check their references on http://www.resellerratings.com
If a dealer is advertising a really lowball price, its a good bet that its a scam. If it sounds to good to be true, it usually is. In these cases, definitely go to resellerratings.com before making a decision. Hope this helps.