A lot of sites are still trying to grow, so there are some good specials out there, especially for first-time buyers. I think auction sites are great for buying unique gifts. You can buy a first edition of Uncle Tom's Cabin, for instance. You'd have a hard time finding something like that at a regular store. You could buy those at online auctions after they'd sold out in the real world.
Auctions are less safe than buying from regular vendors.
I wouldn't buy a kidney. And I wouldn't buy anything from someone I didn't know anything about or something that I needed quickly. Be willing to spend a couple of bucks more if it means dealing with someone with a good reputation.