lol
I've become somewhat addicted to Villager trading... a highly underutilized resource IMO. Mostly just the Clerics and Librarians, though. I can get a couple stacks of glowstone just by bartering a big chest of zombie drops with one Cleric, and with Bobsky's mob trap constantly churning, the glowstone is basically free. And I don't even have to go to the Nether.
If I find a Librarian with a good deal I'll pretty much buy out the store, because I cannot humanly resist what I think is a good deal. It's why Anna doesn't allow me to go with her to Costco and the like. I will buy everything.
Everything. 
So when buying the booksellers out I sometimes run out of space for the goodies. It made sense to put the extras at the Exchange, and the idea kind've snowballed from there.