They don’t sell 1 accessory for 500k gold. They sell 50 accessories for 10k gold.
You need to add a system to check if someone receives gold from multiple sales on the market all around the same time.
Honestly the best system would be to have someone make a list of all the accessories that are actually worth someone (probably less than 50 combinations of stats on items) and if someone sells 50 accessories where none of them are on the “actually worth something” list they get flagged for possible RMT and inspected. Inspect the accessory buyers to see if those accounts look bot like.
Making the list is the easy part. Most players could do it in 30 minutes. Making the system to do all the checks would take a bit of time. I think it’s worth it. If you make it impossible to have the RMT sellers transfer gold to the buyers without them being flagged they will have to leave.