An art gallery functions much the same way that a labor union functions. Both of them seek the best price and conditions for labor. One negotiates with management for a group of workers, the other negotiates with collectors for a select group of artists.
You don't need a professional sandwich critic to explain the postmodern implications of that turkey club you are going to order and art fairs don't need critics to deconstruct the juxtaposition of differing materials among the artists in a particular booth.
Thefts are an occasional, sometimes regular, nuisance for artists and craftspeople who sell their work at fairs and festivals, despite the sincere efforts of the event sponsors and the artists themselves to stop them.