Ingame rules changed to closed legal scam

I saw a level 70 twink today offering 500g to each dps that helped them clear Sunwell. The deal was each member got the gold after the final boss kill. Due to the high gold pay the 70 got to put the loot onto master looter and keep all the drops. Upon the final boss kill, the 70 disconnected.

After complaining to a GM, I got a response of this being a "unsupported transaction". Meaning the twink go NO punishment, and kept ALL the loot. The GM stated the only thing that could be done was to "Blacklist the player".

It is a common belief stating the rules in raid chat at the start of the raid would mean the GM could enforce them making this legal scam very easy to pull off.

I want to suggest a change to the ingame rules to allow the GM to punish a player that breaks a deal when the deal is FULLY completed by the service provider to the end with no hiccups and CLEARLY the payer refused to pay up despite the deal not involving a tangible item.

If I can't get the rules changed, at least someone might become aware of this legal scam.

edit: I call this a scam despite GMs being unable to do anything about it, meaning this isn't actually a scam.