Give this a chance, people. It'll turn out better than you think.
Anyway, if shops had two tabs, selling and buying, you could do away with the artificial coin enforcement. In the selling tab the player would be able to set a limited number of goods he wishes to buy, minimum acceptable quality and the price. The prices and balance would still be expressed in coins, but there would be no physical coins involved, unless the customer chooses to use them (if he doesn't have any goods the shop is buying, for example).