I just found a failure in the Trade system.
When you post an offer to exchange, you only suggests an exchange.
When someone responds to this offer, put his available resources at the mercy of the goodwill of the other player to accept or cancel the offer.
If someone want to hold for pure evil its resources it can do so indefinitely.
It would be safer, that when you send a request to trade, their resources become temporarily trapped, like say the other player has one hour to respond to that trade, otherwise it will be canceled automatically, and no one runs the risk of losing something