Used gear is a very cost effective way to purchase high quality gear for the fraction of the price. I have bought and sold camera gear on ebay and have had a good time, but I also practice due dillegence. These are some tips that have helped me. Many of these apply to other items on ebay, while others are specific to camera gear.

• Check feedback. Just having a high feedback isn't good enough. If the seller only sells $10 knick-knacks and this is their first high-value item, beware. Ebay accounts are often hacked, and reputable sellers can be used as fronts for fake auctions.

• Insist on Paypal. If the seller does not accept paypal, don't purchase from them. When using paypal, pay with a credit card. Paypal does have buyer protection, but it pales to what your credit card company offers. If there is a problem with the transaction, notify your credit card company and they will refund the purchase price.

• There is no such thing as a free lunch. If the price of the used/new body is way off from what you see else where, then it's a scam. Check prices for used gear on various buy and sell forums. That will give you a good idea.

• Double check the fine-print. Make sure it's not some dumbass selling you a "picture of a Canon Dslr" or something stupid like that.

• What is the return policy?

Some questions to ask:

• If this isn't an ebay merchant store (ie a private party), is the seller the original owner.

• Does the camera come from a non-smoking home?

• What is the datecode on the lens? (You can look up the date code and find out the real age of the lens.

• Are there pictures of the lens and body? Make sure there isn't any fungus on the lens, and not major dings and knicks on the body and lens housing.

• If the body is used, find out approximately what is the shutter count.

• Is the gear US or "Grey Market" As @chuqui has pointed out, grey market gear is not eligible for US warranty service.

• If the seller claims mint, then the body should be in mint. Do not budge from this. Any and all cosmetic defects should be explained in the body of the ad.

If your gut tells you something is up, then something probably is, and you should walk away. Also, have you considered looking at Buy and Sell forums on various sites? I can personally vouch for DGrin and FredMiranda.