I was wondering if used books from bookstores were allowed, because they have a stamp on the first page and sometimes they stamp it on top of th book, not the cover but the actual top?  I'm making sure I'm allowed before I post any of the books from there, there are all in great condition otherwise. 

Yes, they're allowed.

This point from the help center is primarily for ex-library but covers the condition of used book store books also:

"Decirculated library books may have various markings and/or stamps inside or outside or on the page edges of the book; this is OKAY.  These may not obscure text."

So just make sure that the stamp doesn't cover any text and you'll be fine.

Thanks!!!