I've been using AB to sell stuff for some time now and although its still quite a small site it is growing and has very good future prospects to become a main player in it's field. I have made a few good sales.

Approximate cost: $free

Auction Bidz is a great alternative online auction site. The site is well run and they are friendly and helpful. Best of all there is no listing fee for sellers and they will keep re-listing your goods until they sell. Just a small commission at the end when you sell.