It's really a judgment call on your part.
Quitting your job is a big deal - regardless of whether you're applying to b-school or not.
If your store is really doing that well, I'm surprised you've bothered to keep your full-time job -- unless your store is your "day job" and your full-time job is your "hobby". In any case, if you're confident that you can do well on the GMAT, go for it and re-take it. As for whether you should quit your job or not, that's something only you can decide.
Follow me on Facebook