Seller Options: There's nothing wrong with a fixed price -- if you're buying a hairdryer or a jug of detergent. But eBay's other options -- like auction, buy it now, and best offer -- can be as addictive as gambling, especially on high-ticket items.

Shopper Experience: One thing that draws shoppers back to eBay time and again is the sheer wild variety of the goods. It's a browser's paradise, and it even has a category called Everything Else that seems to be designed for people who have lost their minds.

Amazon has one big edge over eBay. It's built for a quick and easy shopping experience, so shoppers can pile up a long and varied shopping list in a hurry. Amazon Prime makes it even easier.

So if Amazon is the big-box store of the Web, eBay is the whole mall -- plus Main Street and the big-city downtown -- all without the commute.

--Written by Carol Kopp of Minyanville.