Technically, these PD/PK certs aren't supposed to be transferable. However, they are traded fairly often over in the Coupon Connection forum (one of those "special hidden forums" you refer to) and they're even available on eBay. In general, I would shy away from those since they could be counterfeit and there's no real way to tell if they'll work, plus they almost never will send you the actual coupon, so you don't have any recourse if something goes wrong with the transaction.