I do not have enough posts yet to even view the Marketplace, however, to have some type of feedback system to protect buyers from shady sellers the site really needs to integrate a token for eBay to verify that the eBay account a seller claims is theirs is really theirs. Anyone could pick a random eBay user name and claim that's them.
I've sold on eBay for years, have no expectations of selling any of my Mac's here, but I would be interested in shopping for a good deal on some older Mac's here, but only if I felt protected. Paypal doesn't protect buyers from third party websites, so there's little to no true protection available. But at least knowing that you are dealing with XYZ who has sold 3000 things on eBay would be more settling.
One last thing to keep in mind, and I have no idea if this is posted in the Marketplace, but in the U.S. if someone takes money through the mail but does not ship a product, that is mail fraud and a U.S. felony. But only if it's through the mail, i.e. Paypal doesn't count. I would recommend buyers to pay with a U.S. postal money order when possible and mail it to the seller. The money order can be tracked if it is cashed, and as I said, it's mail fraud if the seller does not live up to the terms of the deal.