You really have to have an eye for what kinds of cars can carry a decent price even with a salvage title, and what damage is considered a total loss, but doesn't actually make the car structurally unsound.
You'll never make a lot of money off of rebuilding cars this way, as you have to disclose that they were damged and document the repairs. The market for them is VERY limited, as resale is low, and insurance for them can sometimes be higher. The appeal of salvage title cars is the fact that they cost the buyer thousands less than the same car with a clean title.
And since every repair is going to be different (some require a lot of mechanical work, others require a lot of bodywork), it's best to work on cars for a while before even attempting to do this, as you really have to have an innate fel for what you're doing to do this well. Yes, it can be that hard.
I've done it, but I was careful to find cars that were popular, but were cheap enough that the work necessary to fix the car would total it, but not really damage it very much. This simplified things. I had to choose cars that were a saleable color combination, as the materials cost to do a quality respray could remove all "profits." ( really, you'll barely get paid for your labor, so it's not any different than working for someone as far as the money you can make on them). I also documented with good photograpns the condition of the car when I got it, all damage, and everything done to repair it, so that the prospective buyers could determine whether the car was sound.