Find a narrow ravine and have one person stand on either side with the goods to be traded. Suspend a log over the ravine from a single rope in the center, so that it can spin freely and one end can be reached by either party. Each party ties their goods to one end of the log, and once both can see the other party has done so, they spin the log so that they now have access to the opposite end of the log with their trading partner's goods. The log only spins when both parties let it, and no one ever has access to both sets of goods at the same time.

This solution requires that both parties can verify that the other party has indeed put up their end of the bargain by tying it to the end of the log, and can do so from the other side of the ravine.

Find a narrow ravine and have one person stand on either side with the goods to be traded. Suspend a log over the ravine from a single rope in the center, so that it can spin freely and one end can be reached by either party. To start, each end of the log is firmly fixed to the ground on either side, so neither party can spin the log until both release it. Each party ties their goods to one end of the log, and once both can see the other party has done so, they both release the log, and spin it so that they now have access to the opposite end of the log with their trading partner's goods. The log only spins when both parties let it, and no one ever has access to both sets of goods at the same time.

This solution requires that both parties can verify that the other party has indeed put up their end of the bargain by tying it to the end of the log, and can do so from the other side of the ravine.