Almost everything requires a value judgment. However, in your example, if you don't think art is valuable, it seems unlikely that you would be an art dealer as

Almost everything requires a value judgment. However, in your example, if you don't think art is valuable, it seems unlikely that you would be an art dealer as you would have neither the training nor the inclination to learn enough about art and art markets to get such a job. So you would likely be doing something else, hopefully something you do value.