A number of factors. I personally find amazon is better than ebay in terms of professionalism and fast shipping time. It's not that uncommon for ebay people to wait days or even a week to ship. I would personally pay more to a professional seller whom I know will ship within 24 hours as opposed to just some guy who may give me some line about how he only goes to the post office on Thursdays so if I order on a Monday I'm waiting waiting 4 days for him to ship.
Another thing as others have mentioned in search ranking. Whenever a product of mine goes to the top of search that is what sells like hotcakes. It's amazing I'll be thinking oh this thing is my best seller and then nothign but something else sells, its all about placement in search.
Lastly, many buyers don't look around. I ave a few ebay accounts and oftentimes price things differently as I get bigger discounts on some accounts than others so can afford to drop prices. I have literally had 3 products on the same page like $12, $15, and $20 and someone will buy it for $20.