Instructions
Identify your card and assess its quality. Research the 10 official classifications and their descriptions. Don't fool yourself into thinking you have a "Near Mint" card when it's really just "Very Good."
Consult price guides to determine your card's retail value. While it's unlikely that you'll be able to secure this price from a retailer, as they pay wholesale prices, you'll start to get a ballpark idea. Respected price guides include Beckett and Tuff Stuff. Several companies also provide free information online.
Compare online auctions. With huge numbers of trading cards bought and sold daily, auctions are an excellent resource for seeing what buyers are willing to pay. On eBay, you can run an advanced search using filters that show completed listings.
Consider current trends. If your trading card features a character or player that is getting lots of publicity, this might positively affect your price.
Factor in the economy. Like it or not, prices are also determined by the public's willingness to spend money. In struggling economies, you might not be able to get as much for your trading card as you might during a boom.
Visit local card shows. Not only will this put you in touch with other traders and collectors, it will help you determine how much buyers in your region will pay. A Braves card would likely bring a better price in Atlanta than it would in Santa Fe.
Make a decision. Once you've compared sales data and factored in the economy and current trends, set a price that will be fair to you and your card's future owner.