The developers can cash out anytime. Since AppCoins is a ERC20 token that will be listed in the different exchanges after the ICO, they can sell or buy.
When a user earns AppCoins by installing a game and providing 2 minutes of attention to the game,  the coins received requires a multi sig to be used.
The two signatures in the multisig of those coins are from the app store and from the user. When the user wants to buy something inside a game, both the user and the store need to sign the transaction. If the user wants to cash out, they don't have the store signature to sell in the exchange, so it's not possible.