To get a baseline for how much your cards are worth, browse the Web sites of auction houses such as Heritage Auction Galleries or Lelands for price listings on comparable cards. Tuff Stuff's Sports Collectors Monthly also has a portion of its price guide online.
Another common pricing reference for collectors is "The Beckett Almanac of Baseball Cards and Collectibles." The guide is also online, although you need to pay for access. Subscriptions start at $6 a month.
Keep in mind that price guides are usually based on cards in good condition; cards with wear and tear could be worth far less.
You could always bring your cards to a hobby store too. A knowledgeable worker should be able to give you an idea of what they're worth for free, so long as you're not hauling in crates of them. Of course, you'll want to double check any casual estimates.
HOW TO SELL
If you have cards worth around $500 or more, consider selling through an auction house. This gives you access to a pool of experienced buyers willing to pay top dollar for quality cards, says Joe Clemens, price guide editor at Tuff Stuff's Sports Collectors Monthly.
Auction houses typically take a commission of about 15%, although that figure can vary depending on the sale price. So for a card that sells for $500, you'd pay a commission of $75.
Cards of lesser value can be sold at hobby stores or baseball card conventions. EBay is another option, although Clemens notes that the overall breadth and quality of cards isn't as great, so bids tend to be lower.
One way to drive up your card's value is to get its condition professionally graded, since this gives buyers confidence about what they're getting.
The two big grading services, Professional Sports Authentication in California and SGC Grading in New Jersey, seal cards in plastic cases, which are tagged with the card's serial number and a grade ranging from one to 10.
The points make a big difference. A rookie Mickey Mantle card with a grade of 8, for instance, was sold by Heritage Auction for $71,700 last fall. The same card with a grade of 1 sold for nearly $3,600.