I don't believe playing in the All-Star game increases merchandise.
The game manual says:
However, we have made it so that no team benefits from the all-star game at all; injuries won’t be carried over to the league games, minutes played don’t count towards training or game shape, and there is no arena revenue from the game.
Merchandise would be an advantage. Also I had a player in the All Star game and didn't notice an increase in merchandise.