It's actually a really good system that has done well in other MMOs (EVE is a good example).
It removes the level of risk that you have when buying gold from a gold farmer, you never really know if you'll get it or not, no matter what guarantee they make. The other benefit is that when gold farmers sell you gold it will come from their constant farming of gold, which increases the amount of gold in the game and thus; inflation. With this method, they are sold to other players for gold that is already in the game, so the players buying the gold aren't contributing in the same way to inflation. This also isn't taking into account the players who get hacked and what not for RWT purposes.
If the gold farmers have to constantly lower their prices it becomes less and less worth it (especially with less customers as people will opt for the safer option) and they will have to farm more gold, and with inflation lessened, farming will be harder, meaning more time for less money. It's as system that has been shown to work in other games already and hopefully will work well in RS.
Sure, it might not stop all gold farming/RWT, but it's a good step to solving the problem.