book stores are idiots if they think trades are better. bigger books take up more shelf space, means fewer copies, means fewer sales. even if they do charge more for a TPB. someone could go in looking for Treason and intend to buy something else and not find Treason, then decide buggerit and buy nothing.
Works in theory, but in actuality, trades do make bookstores more money. The shelf visibility is higher, so people are more likely to pull a trade at random to look at, and they get more money per copy sold, enough to compensate for the books taking up more space.
Hardcovers are so much more that people buy them less despite the greater visibility, but trades hit a spot right in the middle that makes bookstores the most.
(Disclaimer: I don't work in a bookstore, and this might be a misinterpretation, but as far as I know that's how it works. I've had this explained to me before, but I might have a couple details wrong.)