There's a market out there for all but the most obscure players' signatures, though that doesn't mean you're going to break the bank with most of them. Still, if the cards you find are from about 2000 on, it doesn't hurt to check to see if any of them are autographed.
Authentic autographed cards certified by the card companies are a relatively recent development, so be wary if you find older signed cards. Chances are that these autographs, even if they are legitimate, won't be worth much to most collectors because there is no way to know for sure if they are real without using a third-party authenticator - which can be expensive and still may not end the debate.