Above I assume that the short seller can invest all the proceeds from the short sale. Some of these restrictions are that there is sometimes little opportunity to sell short. There are restrictions on what the short seller can do with the proceeds. Usually, the short seller only has use of a percentage of the proceeds. Another obstacle is that some goods may not be easily stored.
Gold is easy to store.
Wheat and oats can be stored for quite a while. However, they can't be stored infinitely. This leaves the occasional retail client in a poor position to take advantage of cost of cost of carry arbitrage. A large firm which has a substantial amount of the commodity, its own trading desk, and its own people on the floor of the exchange where it has a membership, has all the advantages. This firm will be able to take full advantage of the arbitrage opportunities which arise.