The how is interesting because he started with only $200 as seed money:
"It was very easy. If you think about it, the reason that most service companies stink is that you don't need much money to get into them. All I needed was business cards and a phone number. If I did a good job, if I worked enough hours, if people liked me, I'd get referrals. I grew it one step at a time, one employee at a time."
There were two things going on here. First, his business had low barriers to entry. His biggest assets were his brain and talent. Second, he had simple short-term goals. He grew with incremental gains.
At some point, that incremental growth wasn't feasible. He really needed another mechanism to expand Geek Squad. After ruling out franchising and investors, he sold the business to Best Buy. It seemed a natural fit to put the Geek Squad next to Best Buy's electronics. Other businesses were partnering in a way that "one brand could live in another, in harmony."