Self competition?

Someone explain this to me.

I just went to Amazon to buy a friend a book. Amazon was asking $19.25 for the hardcover edition, not bad since I'd paid $27 when the book came out. However, next to the Amazon price were the words "42 used & new from $5.99." I ended up buying a new copy of the book from a dealer in Jersey for $9.75.

Is Amazon raking enough commission back from the merchant I used to justify forgoing its take on a direct sale? Or is this to keep people who sell discount books from nibbling into Amazon's market share?