I used to work for a successful small repair shop. The big thing to staying successful is having competitive prices and better service than the next guy. It's all about making the customer happy. A lot of people don't want to buy a new computer and want to fix the ones they have, and some will be willing to pay no matter what the cost is sometimes (which is a bit ridiculous, but if they wanna do it....).
We also sold refurbished PC's that we would either buy from others off the street or from a large reseller that dealt in refurbished PC's. They were well priced for their specs and very good machines (mostly Dell Latitude's, HP EliteBooks, and Dell OpriPlex's) as well as the occasional Insiron, HP Pavilion, etc.
You just need to be good with customers and not talk down to them. Tone down the tech talk because most of them don't understand the lingo. Also, even though you know they're all idiots, don't let them know you think that. And also no matter how stupid the thing is they want you to fix, just do it. They're going to be willing to pay you.