Thoughts on organizing:
If it's in a parking lot, I'd think maybe the easiest way to organize space would be by parking lot slot

I don't know what his lot looks like, but if he cordoned off one half the lot and rented vendor spaces by the slot (or two slots), maybe that'd work?
Here's a link to the Troy swap meet's vendor application (it's a pdf, just so you know) which might give some ideas:
http://www.troyswap.com/pdfs/Application_2013.pdf I know some of the TSCs hold swaps occasionally, so obviously there's some way of addressing liability issues that works for them, but I don't know how they handle it.
Hopefully the vendor spaces wouldn't be
that expensive, lol! The Troy swap is pretty big and well-established, plus the lot rent and admission is the way they get paid -- I'm assuming he'd be doing it as advertisement for the store somewhat, as well. Speaking of, as far as advertising goes, I'd bet advertising it on craigslist, here, and maybe the local paper and/or Columbus Dispatch would probably do just fine