From the Help Center:
Can I use the club for advertising or to sell anything?
No advertising is allowed on the site.
No advertising of any kind may appear anywhere on the site, including but not limited to Personal Messages, Discussion Forum posts, public profiles, forum sigs, book reviews, Eclectic Pen submissions, Book Wiki and Book Tags.
No advertising may be included with any PaperBackSwap mailiings.
PBS member addresses may not be used for mailings of any kind without the consent of the receiver.
No selling is allowed on the site.
The club is designed to swap books for credits. No items of any kind may be offered for sale on the site, not for money or any other compensation, with the exception of Book Credits, which may be sold between members.
Breaking these rules will jeopardize membership. Depending on the severity of the infraction, one incident may be enough to terminate membership altogether. Normally (in cases of lesser severity), one incident will cause a warning and a repeat will cause account termination. Severity of the infraction is assessed by the PaperBackSwap Team, and penalty is decided in our sole judgment.