Like the postman in Boston only from player to player instead of game to player. Type in the player's "PO Box Secret" and pay a fee to have a message or item sent to their PO box for it to be checked later. Attach a silver amount to the item for delayed trades.
An example being someone messages me on the forums asking for "x" quality seeds for "y" price followed with their PO secret. I log on and send the seeds to the secret with "y" amount of silver as a payment for the person to be able to claim the item. This makes trading safe and convenient for everyone involved and eliminates the need for two people needing to be on at the same time. (Timezones, rl, etc)