First, NEVER take the word of a salesman. You should personally go over that boat with a microscope. Make them rig the boat at the store.
Second,a mylar sail is NOT the same as a dacron sail with mylar stuck on it. The mylar layer takes the place of the stiff coating on a dacron sail. Without the mylar, it would have no shape or strength.
Third, Hobie 17 sails are notorious for delaminating. The cheap fabric has mylar on only one side. A sail looking like that one Will explode on a windy day.