Some player have a talent to do business, the know about the demand, how to speculate the market, to control the market price, doing sales and having a large network.

Some player don't, they just enjoy game like normal..... Just look at EVE online.

Not the best solution to prevent gold seller, example:
SE sell 12.99 € = Token 1 month sub = 100k Gil
Gold seller can sell more cheaper with 10 € = for 100k Gil