Haha I think most people are smart enough to factor in BP, so in your example instead of being scaped $9k they think of it as well "I'm not going to pay that $9k BP but I will pay $55k for this book, so I'm going to bid $46k." If there wasn't a BP then I think hammer price would just be driven to $55k instead. Whatever reason they have for doing this besides collecting more fees or whatever, perhaps there's some tax loophole or back-end accounting where they can mess around with this buyer's premium number to make the books look nicer.