you're in a crowd of a thousand people, most of whom are packing, and you think you're going to get robbed?
Sounds like a very fruitful field for a master pickpocket who has a place to drop his takings. Might pick up a Glock or Beretta, too...
People that carry, have a heightened sense of situational awareness.
If you ever carried, you would be aware of this. That said...
Seems to be scant interest in using/accepting Paypal at a swap meet. If I was approached by someone expounding the many benefits of accepting Paypal. I would politely ask the individual to move aside, make room for cash-paying customers.
To reiterate, Good luck with that.