Unpopular opinion incoming, but the best solution is to drive them out of business.

When I played WoW, I saw gold sellers take a nosedive (at least on my server) when they introduced the WoW Token. It's a way for one player to spend gold on game time, by buying a token that another player bought with real money. Basically buy someone else a month of sub time and they pay you in gold. The market is automatically regulated by Blizzard, no player involvement at all. Gold isn't generated, it's only transferred from one player to another.

People cry about it all the time for dumb reasons, but it really did a number on gold sellers. If someone wants to buy gold, so to speak, they get sub tokens and do it that way. It's legal, so there's no risk. It meant that the gold sellers had to drop their prices to stupidly low levels, because they lost their business.