Old comic books can be worth quite a bit of money if sold properly. The rarest issues can sell for $50,000 or more, and even less valuable books can sell for far more than their purchase price. But for every Action Comics #1, there are thousands of forgettable issues out there worth little more than they paper they were printed on. Selling comic books properly means being able to ascertain what a give issue is worth and then finding a buyer willing to pay a fair price.
Organize your comic books by title and issue, and make a list of each one you have. Individual issues can vary widely in price, and you may be able to get more for complete sets sold in bulk that you could trying to sell them one by one. An accurate inventory will save a lot of confusion later.
Gauge the overall condition of your comic books. Old issues are graded on an eight-point scale: Mint (MT), Near Mint (NM), Very Fine (VF), Fine (FN), Very Good (VG), Good (GD), Fair (FR) and Poor (PR). A book's condition will play a significant role in its price. You can preserve your comic books by placing them in mylar bags with cardboard backing to keep them from becoming bent. Store them in a cool dry place away from the direct sunlight, and handle them carefully, if at all, when you want to read them.
Check price guides to see what the market value of your comic books will be. The Overstreet Price Guide sold by Gemstone Publications and the online Comic Book Pricing Guide are excellent resources to check the current market value. You can also gauge the market by speaking to reputable comic book dealers or by monitoring auctions of comic books on EBay.
Send out feelers for a buyer to see what you can get. You can do this by advertising on forums frequented by collectors and fans, speaking directly to dealers at conventions, posting flyers with basic content info at local stores, or simply opening an auction for them on eBay. For old comics that may be worth quite a bit, you may want to consult a third party such as the Certified Guaranty Company, which can help negotiate a sale fairly.
Sell the books after you have settled on a fair price and are assured that the buyer is acting in good faith. Beware of lowballers or shysters who will try to buy your comics for less than they are worth. You should always negotiate face to face if you can and never turn the comic books over until after you have been promised the money. Again, third-party negotiators can help a great deal: helping both to confirm the fair price for your books and to ensure that the buyer pays promptly.