A/C fan sometimes doesn't blow

My fan will go out, but only when my car has been sitting in the sun. Someone recommended tapping the housing in the passenger side foot well. Sure enough, that has worked EVERY time. It never goes out when it's cold, or even in the hot garage. When it's not blowing, I can feel that there is cold air with my hand near the vent--it's just not going anywhere.  I'm selling the car soon, and don't want the buyer (who has COPD) to have to deal with this.