Hmm, I don't think it's stealing to pay a ridiculously low price for a valuable item. If the seller doesn't know what they have, they are no worse off for getting your $2 for it. Unless you then dance around the store and wave it in their face singing, "Thou-sand dol-lar bo-ok, Thou-sand dol-lar bo-ok!"
I think it would be kind of weird, actually, to say, "I would like to buy this book please. Now, you realize that you are asking $2 for it and it's really worth $1000, right?" Who would do that?
Like hooch, I might tell them if I wasn't interested in the item, but be honest, if you shop in used bookstores and thrift stores you're dreaming about making one of those "few dollar" scores. That's why you walk into those joints.
A while back there was a chance that someone had listed a 1962 copy of Run With the Hunted in an eBay auction along with some other Bukowski books for a "buy it now" price of $60. As it turned out it was not the 1962 Run With the Hunted, but when I hit that "buy it now" button when there was still a chance that it was, I felt not a pang of remorse, I guarantee you. ;)