Is there a policy about old library books?  I have one that was bought at a book sale at a library.  The checkout barcode has a black marker line through it.  Just wanted to make sure we are aloud to trade these.  Thanks!

Yes. They need to meet the standard posting guidelines. Having the slash through the bar code or a stamped Withdrawn is helpful, as it reassures members that the book is legit.

Ex-library books:  Yes

• Ex-library books often have stamps inside or on the binding or on the page edges: this is fine.   If you receive an ex-library book that is not clearly marked "withdrawn" (not all libraries do this), you can contact the library to ask them if this book is still part of their collection.

Ex-library books must meet  the other conditions of postability (no water damage, dirt, etc.). 

But other than that, they are OK to post.