to be honest, no matter what you carry I'll still do the vast majority of my shopping online. With the overhead, it will be impossible for you to beat online prices. Plus with buying online, you don't have to pay taxes most of the time. As for frags, I prefer to get mine from local reefers. What you may charge $35-40 for, I can probably get from someone else for $10-15.
But on a more positive note, that still leaves supplies and fish. I like to buy all of my supplies and fish locally for obvious reasons. I don't want to pay shipping on a heavy bucket of salt, and I don't mind paying more for a fish that I can see and be able to tell that it's healthy.
Good luck in whatever you decide.