Could we:

Send items via mail and accept C.O.D, Cash on Delivery. For example
I'd receive an offline tell about some item I had for sale - I could
go to the post office with the item and send it as a package to the
recipient. The post office would 'id' the item as a postage for and
deduct that from the COD-money. The delivery would take an hour, and
the recipient could go to the office and see that it is a valid item
he's buying. The item would stay in the office for a week, or could
be withdrawn by the original sender for a % fee of the COD amount.

OR:

Give merchant carts anti-blacklists, so that you could sell an item
there to a certain person only. Like I could just tell the offline
person that his item is at my cart in <location> for the accepted
price and nobody else would see the item in the list.

Thank you,

~/A