I do not shoot Leica...

I am not an expert in grammar but having published eight books and many articles in English I think I can answer with some experience.  I find the phrase "I do not shoot Leica" fine.  Language is malleable and constantly changing.  Those rules we learned in school may be good, but they are not engraved in stone.  What matters most is clarity, then maybe tone.  Your point is clear, especially in the context of the conversation.  Whether it is pompous in tone, that seems ambiguous, although an author has responsibility to make it not so -- when writing!  Spoken language is considerably freer than written language.  The store person should not have corrected you language at all, unless he was peeved that you were not a prospective Leica buyer; in that case that is not someone I would want to give my business to.  (If I were writing formally I would say "to whom I would want to give my business.  Now that sounds pompous.)

To use the word "shoot" as in "take a picture," does have a certain ring of machismo and violence, but it is one of those terms that has been so sanctified by long usage that I personally no longer find it problematic.

Michael