How do I print off a mailing label for books?

If you just want the mailing label with the addresses there will be a "print label" button.

If you want to print postage as well as the address you need to buy PBS Money in the Kiosk then you still use the "print". It will take you step by step on what to do. You will need to know how much the package weighs and input the date you will be mailing.

I'm sure the help files can assist you more as well. http://www.paperbackswap.com/help/help_index.php 

On your main page, under 'Books to Mail' tab.  On the lower half of the book transaction box you should see two buttons.  One is labeled Step 1 of 2 and says 'Print Label' and the other is labeled Step 2 of 2 and says 'Book has Been Mailed'.