Hi, I haven't been working as a yacht broker for a long time. But, from my point of view, the best would be to start as a seaman that knows how to navigate and therefore who knows the pros and cons of the kind of boats (sail or power) you are selling. A good knowledge of engines and/or of rigging is helpful too.
Of course a nice approach with people is needed and, last but not least, a good memory for numbers plus the ability to work around percentages (commission and various taxes).
my two cents! :)