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Mr. R. Leinwather's suggestion that (sports) betting is similar to stock trading gave me this idea: long-term bets with online betting services (not the same as going out to the books yerself, but oh well) should be able to be bought, sold and traded between patrons.
Say you've bet on Celtic to win the cup at 2 1/2 to one, and a month later, the odds have improved to 2 to one following the elimination of several other teams. You should then have the option of selling your stake to somebody else for slightly more than you invested. You would then gain a small (but safe) profit, and the other person would be able to buy into better odds than they would otherwise receive.