At some point, for some reason, everyone sells some cards. Your collection might need culling, you might need some extra cash, you might want to cash in on a hot card or series you've been saving or any number of other reasons. The best place to sell your baseball cards is often very much influenced by why you are selling, so bear this in mind when considering your options.
Area Baseball Card Stores The most obvious place to sell your baseball cards is not necessarily the best. Bear in mind that baseball card store owners are in business, not just because they love the cards, but because they want to make money. Some difficult-to-move commons might be easier to sell at a store, who has to maintain inventories and build collections; additionally, if you are holding on to one of the world's most valuable baseball cards a professional can be invaluable in helping you evaluate and sell your card. However, in most cases, better prices can be attained by going as close to the collector as you can -- after all, that's what the baseball card store owner will be doing with any cards he buys from you.
Local Sales and Trades If you're mainly looking to thin out duplicates and find some interesting new cards to add to your collection, your local market is a great place to start. Flea markets, swap meets, ads in the local paper and on Craigslist.org, card and comic shows, etc. will often provide you with no shortage of potential trading partners. This is especially true if you live in a city where an MLB team is based. Networking with other baseball card collectors can also be a lot of fun -- after all, a collection shared is infinitely more fun that one sitting in a box under the bed.
eBay The mother of all online auction sites will provide you with no shortage of potential buyers. The flip side is that it is a highly competitive market -- most major baseball card stores in the US have an online store on eBay, and of course a huge number of amateur collectors and traders are there as well. Take your time to research what is selling at eBay using tools like eBay's Pulse and Want It Now pages; if you want to sell serious volume or start on a regular basis, third party research tools like the Terapeak Price Report can be invaluable.
Forums and Other Specialty Sites There are a great many mailing lists, forums, and other types of sites dedicated to baseball card collectors. Many allow and even encourage members to appraise one another's cards, make trades and sell outright. Among the top websites for selling and trading baseball cards are Sports Card Forum, Elite Sports Card Forum, and Trading Card Central.
Bulk Buyers If you have a large number of baseball cards to sell, especially if most are commons and duplicates, it might be worthwhile to consider a reputable bulk buyer like KrukCards.com Bear in mind that most such buyers are accustomed to dealing with collections that number in the thousands of cards; that said, some might be able to point you in the direction of a potential buyer even if they are not directly interested in yours.
There are many different ways and places to sell your baseball cards. If you want to get top dollar for your baseball cards, take your time to research your options and the current market, as well as evaluating your own aims in selling them.