01.Who sets the price, and who your counterparty is
A bookmaker prices a market, builds in a margin, and manages its own exposure — you are betting against the house. On Polymarket the price is whatever two traders agree on, and the venue earns from activity rather than from your losses. The practical consequence is that a sportsbook has a commercial interest in your outcome and an exchange does not.
- Sportsbook: prices set by the operator, who is your counterparty.
- Polymarket: prices set by an order book; the venue is not on the other side.
- This is why sportsbooks limit consistently winning accounts and exchanges do not.
02.Where the margin sits
Sportsbook odds carry an embedded margin — the implied probabilities across all outcomes sum to more than 100%, and the excess is the house edge. Market prices on an exchange tend to sum much closer to 100%, with costs showing up as a spread and explicit fees instead. Comparing a sportsbook price to a market price without normalising for that overround produces a misleading answer every time.
- Sportsbook: overround baked into the odds, often several percent.
- Exchange: cost appears as the bid-ask spread plus any stated fee.
- Sum the implied probabilities across outcomes to see the margin for yourself.
03.You can sell a position, not just hold a bet
A traditional bet is a fixed contract until the event settles, cash-out features aside — and those are priced by the operator on its own terms. A market position can be sold at any time to another trader at the going price. That turns a bet into a tradable asset: you can take profit as a story develops, cut a loss, or trade the move rather than the outcome.
- Sportsbook: the bet is fixed; any cash-out is on the operator's terms.
- Polymarket: exit any time at the market price, to another trader.
- Most active traders take profit long before resolution.
04.Scope and what you can automate
Sportsbooks cover sport in enormous depth and almost nothing else. Polymarket covers politics, economics, culture and sport with less depth per fixture. And a sportsbook will not give you a trading API — automated betting is against the terms of most. Polymarket publishes its APIs and settles onchain, which is why an automation ecosystem exists around it.
- Sportsbook: unmatched depth of sports markets, no programmatic access.
- Polymarket: broad event coverage plus full API access.
- Automation is a supported use case on one and a bannable offence on the other.