From the red sticky "How to use this forum" thread:
(3) No selling books for anything but credits, and no offering anything but books for credits. Freebies/incentives can be "non-swappable" items, but remember that non-Media-Mail eligible items CANNOT be sent using Media Mail postal rate. For example, DVDs and VHS tapes and magazines and pamphlets/booklets without ISBNs are all NOT permitted to be posted at PBS; they can all be offered as freebies/incentives, though. But if the freebie/incentive items contain advertising (ie, magazines or booklets bought at a supermarket checkout stand), or are not Media items (anything but books, CDs, DVDs, VHS tapes) they can't be sent Media Mail, so the whole package will have to go by another shipping rate. DVDs/VHS tapes/CDs can be sent using Media Mail postal rate.