The only items you should be able to buy with real money are ones you can't get any other way. Otherwise, it's considered "Game breaking". If I can spend $100 and be 10,000,000g richer, it's game breaking. You should only be able to get non-tradeable items with real money. It's the place where most games fuck up, and it becomes a "pay to be rich and powerful" type thing, it's stupid.