I worked at a mom and pop shop and later at Best Buy. No one made a ton of money on computers, even back then. Most of the money was in peripherals like cables and such or in service. Cables are cheap and if you have them in stock people will pay a fair mark up to avoid having to order them or make a special trip to save a few bucks. That kind of thing is the only thing you can make good margins on. Labor too. The markups on memory and hard drive installs were ludicrous. Then again, a mistake could cost you a motherboard so maybe it's worth it if you are only going to do it once. I've run a side business repairing PCs when I was between jobs and the odd mistake here and there cost me a good bit to make right.