Some Chick wrote:
That isn't reasonable to suggest sharing the windfall.
I'd have to agree. If you buy a house and turn around and sell it for double what you paid for it the next day, you're under no legal, moral, or ethical obligation to share it with the previous owner. They say "caveat emptor" (let the buyer beware), but that also applies to sellers in my opinion.
Stolen book? I doubt it. Most people don't understand what makes a book valuable.
For me, unless I had a reasonable suspicion that the book was stolen I wouldn't see any reason not to buy it and resell it. I collect old typewriters (only the really nice, really old ones) and sometimes I'll see one at a yard sale for $2.00 or something like that. A "take-it-away-please" price, if you will. I know it's worth more than that, but I don't feel obligated to tell the owner that I think it's worth a lot more than what they're asking.
That's just me, though.