Auctions typically take place over several days, giving bidders plenty of time to fight over the value of your autographed Cheryl Ladd posters. Buying auction items is just plain fun. If your bid is higher than the current highest maximum, you're the new leader. There are several auction sites on the web. EBay has been around for many moons, and has all sorts of useful features for buyers and sellers.
Most auction sites charge the seller a small fee for each item sold.
Although it's not as cheap as just posting an ad to Usenet, selling items via auction is less work and a good value. It is a whole lot of fun to participate in an online auction, and it's also a blast just to browse the aisles and aisles of weird stuff that folks have for sale. Focused on newsgroups but rife with good tips for online selling in general, this FAQ provides pointers on writing an effective advertisement and tips on making sure you don't get swindled.