That would just cause people to leave the server.
Cirrus brings up a good point, an npc that "eats" up coupons seem to be a good idea. Especially due to a large supply of coupons in the market.
After a few months of such an npc, another npc that trades elite weapons for perhaps ygg tickets or a high amount of coupons should be introduced. This will set a sort of minimum price in which the weapons can be traded for coupons.