I am hoping the new patch simply blocks any email that mentions STOgold or the other annoying sites (I have a few from IGE too) and sends them to a GM for inspection. If spam, insta-ban the account.
Then they need to work on fighting the chargebacks from the credit card companies. (Most gold sellers either outright steal the account via a hack or they use a credit card that they charge back [essentially dispute the charge with the credit card company]. Essentially, the devs don't actually get paid by these folks.) All Cryptic has to do is prove that the charge was correct and that the player was banned. Show them the email and the violation of the TOS. (As a lawyer, I have had to do this before; credit card companies are generally decent folks.)
Then look into anyone who messaged or gave anything to the banned player. Presumably, you can trace goods from the farmers to the sellers pretty easily. Then start banning the farmers.
Of course, once you get this web untangled, you can start banning the buyers too.
Sure, it takes time, but you can make their lives living hell and make them rethink whether this is an MMO they want to deal with. And as a heads on a post deterrent, feel free to show a list of banned accounts on a wall of shame and infamy. Especially the buyers who were banned. That might make people think twice about supporting these scum suckers.