If it bothers you to the point it is making you feel bad then if the check arrives, send it back with with postage spent and enclose a note saying Sorry for inconvenience but the book is no longer available for sale. That's all you have to do. If you do find the book is more valuable then you thought then contact one of the many booksellers at Ebay and let them make an offer. That is the telling, what book sellers are willing to pay for a book to resell. Oh, don't send the book until the check clears! Learn my lesson on that. ;)