Yes, with some stipulations. The staff must be informed prior to any event at which you wish to sell. The materials you are selling must be in some way related to the game (costume, weapons, props, etc). Selling or displaying wares is prohibited while the game is actually going on, so it is restricted to the time before and after the game itself. Obviously, characters are free to sell whatever in-game items they have during the event—for in-game money—especially considering how important mercantile matters are in game.