I bought a marked down book from a bookstore and they took a Sharpie and drew a thick black line on the bottom of the book, from the front cover to the back to show that it was a markdown. does that mark make the book unpostable?

Brandy,

That is called a remainder mark.  Books with remainder marks are postable as long as the ink has bleed through to obscure the text.

I believe that would count as a remainder mark and is fine to post.

Thanks for the quick response!!

Remainder marks are postable.  Here's from the Help Center:

Page edges: remainder marks are OK.  A name written on the page edges is okay (if it does not bleed in and obscure text on the text pages)