That's a common misconception. The main barrier was that the soviet currency wasn't exchangeable and possession of the types that were (so e.g. USD) was illegal without official permission.

There was a chain of stores in the Soviet Union named Bierozka with western goods you could buy with USD - officially for tourists and workers from abroad.

Of course there was a flourishing currency exchange black market, but it was tolerated because it enabled the citizens to get rid of their foreign currency - something which the state wanted.