Many eBay sellers list fixed price items simultaneously with Amazon.com. Craigslist, which is partially owned by eBay, also competes with eBay for selling products over the Internet. While neither of those are auction based they can still be considered competitors because eBay is not limited to auction items, they also compete in the fixed price marketplace.
Perhaps only Amazon has the muscle to enter the auction space. One has to wonder why it is waiting to do so.