At 13, you shouldn't worry so much about selling things to make loads of money. It's not going to happen like that. Right now I think you'd be best off just learning how different types of things go together and what you might best have as far as tools for doing the repairs.

All the advice from the iFixit guys was spot on, as expected. Adding to it just a bit, Not only will helping your grandparents and their friends be fun and most likely profitable, but that's the best advertising you can get. They will tell friends and those friends will tell friends. Once they hear that you fixed a computer - and it works now - more people will come to you. There is no better advertisement than word of mouth from a satisfied customer - so make lots of them satisfied.

Things are also different from area to area. In my area there are so many PC technicians that companies fail constantly just because there is not enough business to support so many techs. My advantage is that I do Macs and have many satisfied customers. Almost all of my customers are Mac users, but some also have PCs and friends with PCs, so I also work on them. Macs pay the bills, but PCs let me go to the nice places for lunch. : ) Above all, do what you have a passion for doing. Do what is enjoyable so you don't burn out just from hating the job.

Hope I don't sound preachy, but my son will be 13 in a week, and I've been talking to him about the same thing, so I have this in my mind already.