Ebay - The most popular, most bids, most merchandise.
Yahoo - Looks like second best to Ebay to me. The place to go when Ebay boots you.
UBid - I haven't had much luck here. Seems better for new stuff.
BuyitSellit - Don't Pay Unless You Sell!! About fucking time! I'll be using this site.
Amazon - Very cool feature to sell previous purchases back. Not very good for oddball stuff. Good for new stuff though.
Sell.com
Bidz.com
Feedback Site - Universal Feedback Checking

Shopping Malls

happymall
NextTag
EDirectory

Adult Auctions

Erotic Bids - HEY! A place to sell adult stuff without getting kicked off the site like Ebay likes to do!
Adult Auction House - 2nd place to Erotic Bids so far.

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