It's never been illegal to sell MCCUUs to non-Marines, Marine Corps policy is that it will not be sold. Technically an enlisted Marine could be charged under arts 92, 108 or 134. Enlisted member's uniforms technically belong to the Government, because on the active side they are 1) initially issued 2) given money to maintain. and on the reserve side they are issued them and survey them when they become unserviceable.
Additionally the wording of the federal law in regards to military uniforms is it illegal to wear uniforms or distinctive parts of uniforms, however this is never enforced and I cannot see a service member being charging for selling them unless it is gross case.