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Gold sellers sell gold and items because it's profitable. If it wasn't, they wouldn't do it.
Why not let everyone in the game sell everything, for real money eBay style? For every transaction, the developer takes a transaction fee, based on the amount that an item or items sold for.
In theory, competition would drive prices down enough that it would not be profitable to be a gold seller as a business. As an individual, it could be very profitable, since the individual would have a low overhead.
Buyers would be less likely to go outside the system, even to get a better deal since the in game system would be convenient and the buyer would be guaranteed to get their items.
There are, of course, issues with this, mostly revolving around credit card security, but I'm sure you could get a company like Google Checkout in on it to make things secure enough.
I can not remember winning or losing a single debate on the internet.