Last time I set up to sell at the flea market, I sold a big box full of Atari 2600 games for $50. It was about 120 games (about 105 different), and there was only one rarity in there (Laser Gates, which I think is like an R6 or something like that). Of course, there were also about 5 or 6 Combats, 4 ET's, and 3 or 4 Pac-Mans in there, too.
My rule of thumb for valuation of lower-rarity (R5 or less) Atari 2600 games is thus:
Combat, Pac-Man, ET: 10¢ for a minty cart, nothing if not.
Other R1s, plus R2s: no more than 50¢, generally around a quarter.
R3s, R4s, and R5s: 1/4 to 1/2 of their rarity level, not exceeding $2.
This system has its flaws, however, as there are a couple R5s that seem to go for more than a couple bucks when they pop up. Pretty much everything R6 and up gets priced individually.
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