This could be the start of an expansion of the shop block. My idea for the upgraded shop block:
It would serve as a holding area, where players could each activate the block and place inventory items in one of the two inventory areas, visible to both players to avoid scamming, along with a credit amount. After both players hit the "agree" button, the items and/or credits are exchanged. If either player hits "Cancel", the inventory is cleared, and appropriate items are returned to their original owners. The shop block will probably need some sort of quick-and-easy IRC interface, so that the players can discuss the deal without needing regular game chat. (Although that is still an option. I don't yet know if you can /tell or /speak/ or /say to a particular player, without others overhearing.)
To avoid underhanded theft, the inventory screen will need to be locked when the first "agree" button is pressed. It would be unfortunate for a victim to press "agree", and then have the thief take all their own items and credits off the table, and then "agree" to take all the victim's offered stuff. Sadly, if someone "agrees" right after something is changed that ruins the deal, they'll have to hit "cancel" and start over. But that's just the cost of doing business with a human.
The shop block builder can hit the "AI" button, and the shop block resumes normal block buying/selling operations like we all know and love.