Trading by basket

Q: ??? Consider a country in which people live in high buildings and buy things from street vendors, using some form of money. Each building is equipped with a pulley in the eaves over which runs a long rope with a basket on it, operable by either party. Is there a system by which a resident and a vendor who don't trust each other can conduct trade? Note that either party may plan to play the opposite role with someone else later.
rec.puzzles, 11 Apr 1996