Obviously, you can make money or lose it if you are not careful. I think the first thing you need to establish is what your car repair capabilities really are? Know your limits. Can you rebuild an engine? Can yo udo body-work? Are you willing to invest in a good set of tools and diagnostic equipment?
Probably the best profit can be had from buying non-running cars and getting them into reliable running condition, but this requires the most skill, investment and effort.