I worked for someone who pissed me off so badly that I had a hard time sleeping and one night, I got up and wrote out what I thought I'd need if I wanted to be in that business. I stopped after three full pages in a legal pad, single-spaced. Recently, I looked into buying a business that was closing due to the owner's desire to retire. I knew the payroll numbers, found out what he was paying for rent, utilities, used the insurance numbers from my agent and decided that it cost too much to consider. The rent was $2800 for over 4000 sq feet, which is cheap. The biggest expense was for the service tech/installer, who was being overpaid, but since he did repairs, the store was able to provide what very few in this market can. I decided that it wasn't gonna happen when I determined that I would have to generate profit of more than $12K/month in order to break even and that's without paying myself.
If you really want to do this- get a part-time job doing it, then learn everything possible about how to do it and how not to do it. If you have sales experience (preferably retail, as long as any training was effective and taught the right things- something I almost NEVER see, these days), that's a good start. One of the most important things about this business is listening to the answers someone gives, but also asking the right questions. Unfortunately, it also requires the ability to read minds and the ability to see through BS. When someone says they don't crank their stereo, yeah, they crank their stereo. When they say they can hear things that are inaudible, be diplomatic.
If you find that you can't sign up with manufacturers, you might consider buying and selling good, used equipment. If possible, find someone who knows how to service it and you WILL find some bargains. I just found a B&O Beogram RX at Goodwill last Saturday. The problem- someone tried to play the aluminum platter for too long and the stylus has been ground to a nub. This wouldn't be a problem but the cartridges are rare and the stylus isn't removable but for $20, it was worth the risk. Look around in your area- I'm sure people are selling things they don't need or want. Estate and rummage sales are a good resource, too. You would be amazed by what you can find.