If you collected baseball cards as a kid and long ago tucked them away in the attic, you might want to do some digging because sports cards and memorabilia have grown into a multi-billion dollar business.
Last year, several cards alone fetched seven figures, including a 1952 Topps Mickey Mantle and a 2003 LeBron James rookie patch autograph, each of which sold for $5.2 million. Also, a 1979 Wayne Gretzky rookie changed hands for $3.75 million. But the top sale last year belonged to a century old Honus Wagner card, which sold...