I think that having T3 or T4 wouldn't stop buying or selling from 3rd Party websites.
We're just trying to put more speedbumps in the road. Before, you could buy gold with stolen card numbers on throwaway accounts, transfer it away and then you don't care if we ban the account because you just make another. Now, every time we ban an account, the criminals need to grind up another one to T3. It's not a total solution, but it's just another thing we can do to make their work more difficult while inconveniencing legitimate players as little as possible.
Just make sure you are prepared to deal with bots grinding accounts to T3 when these changes go live.
Is botgrinding really a Problem compared to gold Sellers ? I guess bots are a lot easier to detect too.