Santiago learns a bit about Personal Legends and omens from the crystal merchant; however, I do not think that is the most valuable thing that Santiago learns from the crystal merchant. I think the most valuable thing that Santiago learns is a lesson about the importance of action and actually seeking out his dreams and Personal Legend. Through the crystal merchant, Santiago gets a very good example of what a person looks like and/or becomes when dreams stay dreams and action is never taken to attain the dream. The crystal merchant has his own dreams, but he is unwilling to actively do anything about them, and it is for a silly reason. He's afraid that if he realizes his dream, he will have no motivating reason to keep on living:
Because it's the thought of Mecca that keeps me alive. That's what helps me face these days that are all the same, these mute crystals on the shelves, and lunch and dinner at that same horrible café. I'm afraid that if my dream is realized, I'll have no reason to go on living.
He would rather have the dream always present instead of actually achieving it; therefore, he is always able to come up with some kind of excuse to not make his pilgrimage. Santiago almost takes this lesson to heart because he considers returning home to watch his sheep; however, Santiago has learned that the crystal merchant is also a fairly unhappy individual because of that attitude. Santiago finishes his time with the crystal merchant motivated to continue on his journey.
Santiago learns two valuable lessons working at the crystal merchant's shop. First and foremost, he learns the importance of doing whatever is necessary to follow your Personal Legend. Santiago wants to go out into the big wide world and travel. He is impatient with life and wants so much more out of it. The crystal merchant also has his dreams. But unlike Santiago, he is not prepared to do anything to make them come true. His Personal Legend will remain unfulfilled. The crystal merchant stands as a warning to Santiago: this is how he will end up if he does not continue the active pursuit of his Personal Legend.
Santiago also learns that a stubborn, unthinking adherence to tradition can hold people back. The crystal merchant lives a passive life--only doing something if "it is written." Santiago, however, understands the importance of taking hold of one's destiny and shaping the future in accordance with your own innermost drives and desires. Santiago learns from his encounter with the crystal merchant that fate is not written, and it is not set in stone; it is something that you yourself can create and change.
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.