We could even expand upon the idea of them not being available for trade to people who are not on your friends list, that way the idea of "cultural exchange" is very much kept in tact for those who wish it to be.

This would lead to less cases of people simply scalping the prices to take advantage of others, and actually give reason for us even having a friend's list, making use of a feature that has very few practical uses outside of quick way to send a tell or to find them. (We know we all do so from time to time)