While it is possible to negotiate trades ahead of time through chat and forums, it would help a lot if it was possible to put a trade you are interested in up and set how long it would display, up to say 24 hours. 5 Minutes is irritatingly short when the process of creating the trade lasts 30 seconds anyway, not to mention the impossibility of getting in touch with people who play at times different from your own (except for, again, forums which isn't ideal.)

This of course would lead to lots of trades showing as pretty much everyone would want a trade up at all times, and as such another thing that would be to have a pop up trade window where trades could be far more easily navigated than within the chat interface.

Another possibility is allowing the upgrading of the branch office, where the upgrades allow increased quantities or durations of potential trades.