Hobbies And Interests
Online Price Guides
There are a number of price guides available online. Video Game Price Charts, Atari2600.com, and AtariAge's "Atari 2600 Rarity Guide" are all good places to start looking up your games. Listings can vary from source to source, so the prices you find should be considered approximate value.
Comparison Shop
Perhaps the best way to determine a game's value is to abandon price guides and see what people are actually paying for the game in the real world. Check eBay, Amazon's Marketplace, Half.com and any other site that sells used and collectible games to see how certain games in certain conditions are faring in the market.
Have Them Appraised at a Local Game Shop
If you have a used video game shop in your town that sells Atari games, call and ask for an estimate. Be aware that such places are businesses and need to make a profit themselves, so their quote will likely be lower than what you see in price guides or in online marketplaces. Most of them are fair, though, and making them the "middle man" saves you the time and energy of listing, packing and mailing all your games were you to sell them yourself, which can easily be worth taking a lower payout for them.
What Someone is Willing to Pay
Ultimately, a game is worth whatever someone is willing to pay. If you have a number of games and just want to get rid of them all at once, come up with a per-game cost (consult price guides to get an average). If you have 20 games and decide $5 each is a fair price, put an ad on Craigslist or in your local newspaper advertising 20 Atari games for $100. If nobody responds, consider lowering your price and posting again.