They mark the balls so that they can retreive them and negotiate a deal with the fan who caught it. I, normally am the type of person who holds onto things. I dont sell things of value, I keep them because I enjoy them. However, I would take a picture with the ball and then milk that ball for everything it's worth. I'd get a bunch of autographed s***, try to get season tickets (provided I lived in SF), just a bunch of cray stuff.