Never understood it myself.
Pile your car full of crap.
Take it to a swap meet.
Pay to get in.
Sell nothing.
Pack it all up again and drive home.

Of course if you take decent car bits and put good pricing on them you'll sell them. But as for the tea cozies, old china, buggered chainsaws and empty Coke bottles....

It's not Ebay that killed the swap meet. It's the idiots they allow in with averything other than car/bike parts.