Strictly speaking, Michael Buffer is not a sportsman, in that he has never played professional sport at any level. However, he makes his money from the world of sport, which we think qualifies him for this list. Plus, it's our list anyway, so we get to choose.
Buffer has, amazingly, made every dime he ever earned by saying five words, and then copyrighting those five words so he gets paid every time someone else uses them. If you've watched big time boxing matches, you've seen and heard Buffer start the proceedings with 'Let's get ready to rumble!'. He only took on ring announcing as a job because he was struggling to find work as an actor but finding a catchphrase that fit the moment perfectly has brought him a fortune. He's been paid up to $1m a time to turn up and say it at the start of world title matches.