@AC (13:57): That's already how the public domain works. These guys CAN sell books, and get whatever they can for them. However, as much as they may want to pretend, they are still not copyrighted* and the purchaser CAN just copy the whole book and sell it too if they want.
I don't know what Google's problem is that they won't publish clearly public-domain works, but that's left up to them. Anyone else is permitted to scan or copy the entire book; if they don't wish to, we are just back to the pre-Google situation.
*New cover and interior art would have a fresh copyright, though.