Simultaneous exchange booth

It has always troubled me slightly that for exchange transactions one might person might hand over the goods, then the recipient refuse to hand over the money. So I thought about how you might enforce transfer.

The diagram below is a top down view of what I have referred to as a simultaneous exchange booth. There are two boxes set in a wall. The boxes have a perspex hatch (in brown) that allows the trade partners to see what is being exchanged. The hatch can only be opened if both doors are shut, and both parties simultaneously depress the levers.

This mechanism also allows blind exchange: all you can see is what is being exchanged, not the person.