I think that for items to be traded, they must be equal in a new form of value, made up just for the trading system. Perhaps make it based on public demand. For example: You could not trade a No Land Beyond for a Gjallarhorn because No Land Beyonds Trade Demand value is 2/5 while Gjallarhorns is 5/5. You could trade a Hawkmoon for a Gjallarhorn because both trade values are a 5/5. That's just a BS example but I think that would need to be implemented to prohibit people from trading away their useless exotics for more desirable things.