If you were open about it, I'd expect the organisers would charge you a pitch fee, and they're not cheap. If you were just selling out of your boot in the car park, you'd run the risk of upsetting the organisers with a heave-ho and a vehicle ban from future events run by the same organisers. Many years ago I'd pick up small jobs like event parking and we were all told to watch for that sort of thing going on. The Boinger Bash sounds like a possible outlet, as mentioned earlier, but agreed, you'd need to run it by Mick.