Due to EU and US laws on virtual items purchased with real money, you will never see this. It can be done, but the record keeping, purchase tracking, and government reporting is not worth the trouble for the gaming company. That is why almost all of these Chinese games converted to English by a "US" publisher often have the player to player "trading" systems that are available in the Chinese version disabled.

The way the laws are written (at least the last time I looked into them), it doesn't matter if the items can be gotten for free. In this game any item that can be bought with ingots would have to be excluded from the trading store.