As far as choosing an area to focus on, I strongly suggest that you select an area or subject that you know something about, maybe its an area you collect in yourself, or a subject that you know is highly collected or sought after and is not over saturated with booksellers already. Some booksellers may see choosing a specialty as too limiting, but I see it as just the opposite, as an opportunity to expand one's business and carve out a niche in an otherwise general marketplace. Dear Dave and Biblians, Your point is dead-on. Also, contrary to many postings, nobody is in this business for long if the object is to get rich. When I started on my own, I decided I would only handle what I was interested in.
Life is too short to trade time for money.
Better to trade it for books, but only about things that interest you. Best to trade it for friends, but, again, mostly folk of similar interest. Now most of my correspondence is with people I like. I can't imagine dealing with people who like cars, or rugs all day. Sure am glad there are others to refer them to though.