It's like 98% hobby, 2% business. Baseball is my favorite sport so I enjoy collecting baseball cards (and racing cards, too, as NASCAR is up there) but I don't go looking for cheap inventory to resell but rather sell unwanted cards that might have some value from boxes I open or sell parts of my collection I"m not as enfatuated with that I once was and see if I can get a few bucks that way.
That's a cool Mattingly story you told. I pulled a Mattingly autograph from a box of cards once but am keeping it in my collection as he's one of the rare Yankee players that I admired.