If you can't beat em join em eh?
Price will ultimately determine where most account buyers would go. The forum would basically be competing with the farmers. If the farmers offer better pricing than the forum enough people will still buy accounts that way.
My original suggestion was to remove signatures all together and the only way to get one was to buy the membership. That would have killed farming almost 100% because it's impossible to get a signature any other way. Farmers wouldn't really be able to sustain the market even as it is. It's already incredibly time consuming to farm accounts and has to be done so over a long time (1.4 years or whatever for a Hero) and because of the merit system it probably isn't going to be worth doing en mass. Now people will probably be forced just to having the one or two accounts to concentrate with. Buying an account is also still a very shady market and one where you're likely to get scammed. Most of the people selling accounts are either straight up scammers or are selling hacked accounts and if nobody is going to really escrow them that increases your chances of being scammed or the deal going bad. I'm sure the vast majority of people would just rather create their own legit account with a name of their choice and then purchase a donator rank rather than just buy one off the black market to save a few dollars *and risk getting scammed).