I sold once a few years ago. Honestly it was a bunch of crap that GS was throwing out. Most of the discs looked pretty good, but the titles were nothing special. I took some other junk that I had also found. When it comes to the flea market crowd, believe me they don't care about anything but the games... and store displays if you have those.
I sold the games at about $3 a piece unless it happened to be a premium title then I would ask $5. People haggled, mostly kids and not collectors, and I accepted if they bought a bunch. I made about $180 in a day selling $3 games and a few displays. Getting ready to pull out the rest pretty soon and have another yard sale.
My advice is price to sell, especially if you have a lot of goods you want to move. Great prices make sales more than anything. Don't worry about market value, a profit in hand is better than the lingering potential of bigger profit down the road. Besides, no one at a road-side sale wants to see retail prices on anything, and you might not even make much more than your booth fee back at the end of the day doing that.
As a buyer, I have so many flea market memories I wouldn't know where to start. Used to come back with bags full of games and maybe a $20 bill spent. That's just how it was back then. Found so many great things at those places... I remember when I first got into OG XBOX collecting, I found a guy at the market selling his entire XBOX collection $2 a game, and mostly top tier games. Once, I found a lot of nearly 30 boxed NES games and bought them all for $40. Then there was the time I found a Vectrex console for $20. There was this one guy we regularly bought from, he had milk crates stacked to the ceiling with NES games and most of them were less than $10 each. There was the one time I found an SNES with Diddy Kong 2 and 3 and the guy sold it to me for $5. Once I actually found some cardboard box Sega CD games for like $3 each, and I also bought the entire Prof Layton series on DS for about $10. My brother bought a box of SNES games including EarthBound for $20 once...
It goes on and on. Sounds like a fantasy now, but it's just an era gone by. Sadly not worth the effort to go look anymore, at least for me.