You know what? Buying gpotato and selling them ingame gives you nearly the same amount of money ;)
I did some calculating related to that topic. Buying gp for 20€ (yes, I did this on german Horus Server, you get 200m for 20€). Well, the gp/rupee course was like 1000gp = 95 - 100millions. Now, double that since you used 20€ instead of 10€ and you reach a value of 190 - 200 Million rupee.
It's legal, so you can't be banned, and you make the same win ;)