I personally don't see the problem with sub tokens in trade. If you look at the prices for buying and selling, they haven't changed significantly with the introduction of sub tokens. The tokens aren't worth so much more than in game items that they the only things worth trading with. In fact the sub tokens are priced such that they really aren't worth it for the most part.
With that said, if someone wants to pay real money for progress, let them. Especially since they aren't getting any advantage that someone else doesn't already have.