See also: ZapCon 2014's Game List Is Revealed
If you're new to buying vintage video games, there's a few things you should know. First, trade-ins are done on a case-by-case basis, so feel free to shop around or make a counter offer. That said, you can generally expect stores to offer you around 40 percent of the game's current resale value in trade credit or a much smaller percentage in cash.
Second, the values of certain games tends to fluctuate over time and are affected by things such as trends in nostalgia and whether or not the title has been re-released on newer platforms. Generally speaking, most Atari games can be had for a couple of dollars. Games for the later platforms, such as the original Nintendo, Super Nintendo, and Sega Genesis start around $5 to $8 for licensed titles (i.e. ones based on movies), with popular first-party titles (Mario, Sonic, etc) starting around $10 to $20. Games in the "greatest games ever" canon (Phantasy Star IV or Shining Force II for the Genesis; Super Metroid and Secret of Manafor the Super Nintendo) fluctuate in price the most and can run anywhere from $25 to $50 on up to almost $200 for the rarest of gems (such as Super Nintendo's Earthbound).
Game Over Games is tucked into the back corner of the nerd haven that is AZ Collector's Marketplace. The limited counter space means they don't have the quantity of other stores, but the quality makes it a must-visit, especially for collectors and old-school die-hards. You'll find a great selection of rare gems like Earthbound (if you're lucky) and popular classics for the "standard" consoles (Nintendo and Sega through current-day), but it's this store's selection of the ancient and the odd-ball that makes it truly special. In addition to the Zaxxon machine shown at the beginning of the post, we saw a TurboGraphix 16x, an Intellivision, a host of rare accessories, and perhaps the best Atari collection in town.
For current games, the owner will pay cash for what Gamestop offers in trade, and if you can beat the owner on the Marvel vs. Capcom 2 machine, you can get a 20 percent discount. Even though it's a smaller store, Game Over Games goes toe-to-toe with Flashback Games and Fallout Games when it comes to price.