Why would you even want that bad idea I would say its much simpler to make an appeal on the forums and get it dealt with.
I don't see a plus side to unbans being purchasable. even to someone being falsely banned. If one could buy unbans there would definitely be a limit to the amount of times it can be purchased. The price would most likely be higher each time one was bought. and ultimately at the end you wouldn't be able to get unbanned even if you had a string of bad luck and kept getting banned falsely, but were able to prove that you were not in the wrong. Our current appeal system seems a little more fair to me than if it was just a purchasable feature.