eBay?

eBay is the world's online marketplace; a place for buyers and sellers to come together and trade almost anything.

Here's how eBay works:
*A seller lists an item on eBay, most anything from antiques to cars, books to sporting goods. The seller chooses to accept only bids for the item (an auction-type listing) or to offer the Buy It Now option, which allows buyers to purchase the item right away at a fixed price.
*In an online auction, the bidding opens at a price the seller specifies and remains on eBay for a certain number of days. Buyers then place bids on the item. When the listing ends, the buyer with the highest bid wins.
*In a Buy It Now listing, the first buyer willing to pay the seller's price gets the item.