I'm quite certain the desire for positive feedback is on both sides of these auctions. The seller wants positive feedback every bit as much as the buyers. The first link even states that their feedback system is automated and buyers will receive positive feedback once feedback has been received. (and I would assume only for buyers who leave positive feedback) Unfortunately, he doesn't seem to have any pennies for sale at the moment...
They may have calculated that as a cost for increasing their feedback... might be some buddies in the local area working together.