Qualifications
If you've got enough money to buy a space and invest in a couple of pianos, you're technically already qualified. However, just to be on the safe side, you'll probably want to get a bachelor's or associate's degree in business management. You don't want to jump right into owning your own shop without knowing the lowdown on taxes and the other boring things that are necessary to keep your business afloat. Those two or four years spent learning the ropes will be well worth the money.
The other things you'll want, besides a giant pile of cash and a business degree, are: a working knowledge of pianos, the ability to read sheet music, killer bookkeeping skills, and a friendly, helpful demeanor that just screams, "Buy your next Steinway here!"