If you are planning on selling auto sound systems, you must also have a competent installation shop. I don't think it would make sense to try to only sell gear. For one thing, the big money is in the labor to install the gear, not in the gear itself. With complex integrated sytems (audio, phone, navigation, etc.), it is not that easy for most people to do their own installation, which is where your shop could provide value. You don't want to become the car audio equivalent of Best Buy--a place for people to look at gear, but not purchase. Particularly bad would be offering gear for sale that you will not be installing that one can find in a Crutchfield catalogue or some online equivalent.
Most towns of decent size already have car audio shops, which will make it hard for a newcomer to be competitive. I would bet that most of the work those shops do involve installing systems that are meant to be really loud, rather than catering to audiophile concerns with quality of sound. Perhaps, you could offer a quality alternatives. I don't know what brands qualify in that department these days (Alpine, Boston Acoustics?).
As a lot of posters have noted, road noise is a big problem with car audio. Your shop should put an emphasis on sound dampening products and installation. I have a fairly quiet vehicle (2011 E350) and I still have issues with noise.
Good luck on your endeavors.