What lessons does Santiago learn by working at the crystal shop in The Alchemist?
One important lesson Santiago learns is through the words (and inaction) of the crystal merchant. The crystal merchant, when asked by Santiago why he never travelled to Mecca, answers that "having the dream of traveling is more important than the actual accomplishing." Santiago witnesses firsthand the results (or lack thereof) of not fulfilling one's personal legend. This helps to motivate Santiago to fulfull his personal legend.
Santiago also learned that he needed to be persistent in fulfilling his personal legend. Santiago had to "put his time in" at the crystal merchant's shop. He had to endure the frustration of working for someone else in order to solidify his desire to realize how important it was for him to move forward.