Just look at how other MMOs handle this... in fact, everything I list here is being used elsewhere, e.g. in EO.
Step 1: Allow negative money balances up to a certain amount, e.g. -10'000'000 gil. The hard cap for actually buying items should remain at 0 gil.
Step 2: Each /tell to a character who is NOT on your friend list, in your LS or FC, or an reply to a /tell the player has received, costs a fee of 500 gil.
Step 3: Collect the AH fee when an item is put up for sale, not after it has been sold.
Step 4: Adjusting the price of an item in the AH results in removing it from the AH and putting it up again for the new price, thus collecting the fee again.
Step 5: when the Task Force realizes that someone bought gil or items for real world money, then do not give him a temporary ban. Instead, confiscate the entire sum, plus a second time as a punishment (see step 1).
Step 6: Make gil untradeable, respectively allowing gil transfers only within the free company a player is in.
And then fix the markets and stop punishing your customers which are interested in crafting, please.
Kind regards,
Sol