At a Swap Meet: If you are a vendor, you have a space, on the lawn or on a table, to show your stuff. People (the buyers) wander around and if they are interested, bargain and pay you for your stuff in dollars. You don't actually swap, except for cash. To do otherwise would be to complex. I have a fender, but you have a piston, etc. That is why they invented money. Too bad money is getting "cheaper" all the time (because of inflation).