function sellItem(progress, itemUuid, quantity, price) {
    const owned = progress.inventory[itemUuid];
    if (!owned || quantity > owned.quantity) return progress;

    return {
        ...progress,
        coins: progress.coins + quantity * price,
        inventory: {
            ...progress.inventory,
            [itemUuid]: { ...owned, quantity: owned.quantity - quantity },
        },
    };
}