If SE REALLY wanted to stop people from reposting things on the MB, they'd tags items with 'Untradeable' after the first purchase. Crafters could buy all the mats in the world they want as a newly crafted item would still be sellable.
The only immediate problems I can see with this system is how SE requires stacks of items to be purchased completely. So if a person needed 1 of something, and the only options on the MB were stacks on 50+, that would leave them stuck with items they didn't want and couldn't resell.
So we either keep the status quo we have, or SE has to rework the entire MB and item trading systems, and probably fudge up the economy as well.