They do check IPs as well as any accounts "associated" with the account in question (based on trade logs and who knows what else). Remember that they can look through all the server side data to try and identify botters.
With new leagues comes new server sided detection that no one is aware of, until people start getting banned and a pattern emerges. If we start seeing patterns emerge of people getting flagged, we'll try our best to identify it if it's something we are doing and change it. However, GGG is pretty vigilant when it comes to tracking down RMTers, especially.
It's just the high risk/reward of botting in this game.