A player paid online and didn't show up. Is the money mine?

Yes, if you do nothing: his online or wallet payment stays yours like any paid spot and reaches your wallet after the game.

If you mark him “Didn’t show” (“Manage participance details”, “Didn’t show”), his payment goes back to his Playbox wallet (not his card) minus the fine you choose (0 or 5 to 100%). The fine reaches you as in-app credit you can’t withdraw, minus Playbox’s 7% (at least $0.50).