I have a few books that I bought years ago from a half price book store.  They are in good condition and meet all criteria for a postable  book.  The one thing that makes me wonder if it's postable, there is an ink stamp measuring 1"x1/2" on the inside of the cover or on one of the first pages that reviews for the book are written on that says the name of the book store I bought it from.  Does this small ink stamp make it non postable?  Thanks!

From the help documents:

• 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.
• BookCrossing books may have stickers or stamps or markings on the outside or inside covers or page edges of the book; this is OKAY. These may not obscure text.

While this doesn't specifically state "books from half price book stores" I would say that it still applies. As long as it doesn't obscure text, I would say it is postable.

Thanks for the help!

marks on the inside of covers are fine unless it is a liquid stain, mystery stain or dirt.

ink, pencil, marker, crayon, and ink stamps and stickers are all OK.

unless there is so many marks that the book might fall into the unpostable classification of "excessively worn". Certainly, having a single bookstore stamp does not.