"In great condition...for it's age." This phrase should NEVER be written when selling a book! This is a subjective statement. The buyer, on the other hand wants to hear the objective description of the book. If it tumbles out of it's detached covers when I open the package, I'm going to be ticked, if the seller said it was in great shape. Damage is damage, whether it's in an old book or a new one.

Your word (measured in feedback) is your best selling tool. Be accurate. Give enough information so that the buyer can make an intelligent decision on the purchase. Title, author, publisher, illustrator, edition, copyright and print date (2 very different things). Itemize damage - corners, spine ends and spines are all subject to bumping and wear. Any torn pages? Any writing? Cracked or broken hinges?  Any loose pages?

When a buyers' expectations are met, you have a repeat customer.