I recently had to rush order a whole mess of books for my sister, who's studying abroad and needs me to postal-relay books for some research she's doing. So, I hit the usual suspects --- Powell's, Half.com, and Amazon --- and ordered them. Powell's and Half.com orders both went off without a hitch. However, if you've shopped at Amazon lately, you'll probably have noticed that Amazon is increasingly functioning as a sort of "alternative eBay" clearinghouse, rather than a retailer in itself. Search for any dozen items on Amazon, and at least half of them will be offered by other vendors through Amazon, which acts only as an intermediary.
Most Amazon vendors have reasonable service, albeit not quite as slick as Amazon's itself. Abebooks does not. Abebooks is itself an intermediary for many independent book resellers, and the one book I ordered through Abebooks (from "Moth Y Monarch Books" in Pennsylvania) is the only one that still, two weeks after I ordered it, has not arrived, even though I ordered expedited shipping. (Curse you, Moth Y Monarch Books, and curse the horse you rode in on!) No doubt there are many perfectly good bookstores selling through Abebooks, but ordering through them is a crapshoot.
We tend to think of Amazon as a retailer, but Amazon's moving towards being strictly an intermediary and technology provider, and "outsourcing" the actual work of retailing (i.e., stocking and shipping real physical stuff to real people) to other businesses. I wonder whether this won't hurt them in the long run. Amazon's not the online retailer with the best prices, nor is a search through Amazon's catalog as comprehensive as an eBay + Froogle search. Amazon's main competitive edge is its superior service. Amazon says over and over that they "expect all sellers to maintain the same high standard of customer service that Amazon.com does", but clearly that's not true --- if it were true, then Abebooks, which consistently gets about 20% negative ratings, would have been kicked off the Amazon roster long ago.