Slogan Slingers, the site where you can compete to have a company purchase your slogan for up to $999, used to just have contests that one writer could win. But now they've changed the rules slightly, and if a company likes more than one slogan, they can purchase additional slogans for $50 each. So even if your slogan doesn't win the contest prize, you may still be able to sell it. (But you have the choice whether or not you wish to do that.)
If you haven't signed up for Slogan Slingers yet, check it out -- it's free to join.