Legality in California
Both are CFTC-regulated event-contract platforms, legal in all 50 US states including California. Neither has a regulatory advantage over the other for California residents.
Markets and coverage
Sleeper covers game-level markets (winners, totals, spreads-as-probability) across NFL, NBA, MLB, NHL, and soccer. Chalkboard specializes in player props, individual player performance contracts, with the deepest coverage in the NBA and NFL. Sleeper's player prop coverage is growing but significantly narrower than Chalkboard's.
Pricing & fees
Both charge per-contract fees disclosed at order entry. Neither has a house margin. Effective cost is similar on comparable markets.
User experience
Sleeper's interface is fantasy-forward. Your leagues and group chats are part of the experience. Chalkboard's interface is player-grid-focused. Dozens of player lines visible at once, optimized for rapid prop trading during games. Different audiences, different designs.
Who should use which
Use Sleeper if: you want game-level trading, you already use Sleeper for fantasy, you want a social layer to your trades. Use Chalkboard if: you trade player props specifically, you follow individual athletes more than team outcomes, you want the deepest NBA and NFL prop coverage available.