I dated a guy who did this for years. It was extremely profitable (he loved emachines - he'd buy power supplies in bulk for those things). At the beginning he was also buying modems and such in bulk and selling them - this was back in the early 90's.
But as computer prices started going down, people were more willing to just buy a new computer than get an old one fixed. He sold the biz and now runs and interstate trucking business.
He said the worst part of the job was finding the amateur porn pictures of the owners (this was a small town where you knew everyone) on the hard drives. He also said the religious nuts had more porn than anyone else
