I have looked into it, you would benefit from more research. You will find mark-ups vary significantly from item to item. For instance a diamond may be something like 300%, other items may be in the single digits. Also, you may pay a 100% markup to buy a single item, but only 15% per item if you buy ten of them (the bulk discount effect). Often in such bulk purchase there is a discount from the wholesaler that is passed on to the consumer. Your broad initial statement missed many of these differences.
Comparing to retail AD is not a good analogy. Not only do they have enormous brick-and-mortar costs, but they also sell on a completely different model. I don’t have the actual numbers on jewelers, but I am guessing they sell only one watch every five years or so to a repeat customer, compared to many buyers here who purchase 5-20+ a year. The point being they are closer to a costume jewelry store where people make many, smaller purchases than a single, large-purchase jewelry store. I am not saying 50% markup is not reasonable (and I am quite aware that many dealers sell watches here at much higher markups). My point is that it is in no way standard for all goods. There are many, many factors that pull both ways on any item and sale – enough to discourage such a broad sweeping statement.