I sorry but what a crock, thats a poor excuse for not wanting a trading system. Cheaters, they are here and will be forever.
Manipulating the trading system is easily overcome by setting a limit to number of trades or value of trade per day/week.
Plus theres no need to trade ptp, they could make the system so you have to trade to and from tynon for set values.
Player A trade to tynon player B buys, values set by tynon dependant on how many are being bought/sold.