The same real-world event is often traded on several venues at once (Polymarket, Kalshi, Betfair, sportsbooks). When one venue implies a meaningfully different probability than another, you buy the cheap side on one platform and sell the equivalent on the other, locking in the gap. The edge comes from price differences across markets that should agree; you must convert each venue's odds to a true probability after fees and the bookmaker's built-in margin (the 'vig').
Markets on Polymarket where cross-platform arbitrage is the natural play:
