How about if they are able to detect the gold seller?
Let's say the gold seller has an account worth 500M. He then goes out and "trades" increments of 5-100M to say, 20 different accounts over a length of a few days. If Jagex looks at each of those 20 accounts, they would notice that none of them had known the gold seller personally (e.g. not on friends list) and those twenty accounts would most likely be the gold buyers.
Even in staking, who's to say jagex doesn't have something similar? One large account staking away 500M in fights that he had no chance to win, didn't bother to try.
I believe that if Jagex was so quick to bring back Free trade and RWT then they must have something that is just as effective as the trade limits (which were ineffective anyway) to catch RWT/stop it. If they didn't, then Jagex really has me confused.