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Well an easier way is to do something similar to the items. For items they may be tradable or untradable depending on a value set in the database column 'tradable' in table prefix.items. The trade machine will scan for items that are not tradable, and block trade if this happens. Look for the code at trade.php and related class files such as class_trade.php and class_tradevalidator.php. They can give you some hint.