Umm, I see one hitch which could be severely abused.... Currently, the deal with Legacy members is, we pay less and even get free character slots for our accounts. Seeing as legacy members would be paying LESS cash for the ability to sell their subscription time then little billy who pays 5 bucks more for the exact same item.
OR, a larger problem. We bled for this game and earned legacy status... and little billy can pay bobby 12 bucks to over paypal to buy sub time for him so he doesn't have to pay 15.