How much fixing did they need on average? Just some maintenance and mods? Full restorations?
For the most part, basically just buying something for what I knew was a good price, then fixing whatever mechanical ailments they might have had and sell them for a decent profit. It does help when you can honestly say "Yes, everything works".
In one case I'd bought a '65 Cadillac because I wanted one. After putting a new top on it (a first for me, not fun or easy, but a great learning experience) someone offered me way more than I'd paid.
So I bought a cheap '66, but didn't really like the color. And I much prefer the '65 styling, even though they're quite close.
Got about twice what I paid for that one without having done anything to it - other than adding scrapes to the bottom of it by driving around on fire roads having fun.
A couple of times I got free cars from neighbors, like a Rambler American DeLuxe that had sat for over 10 years. Fixed the brakes and got decent money for it.
Bought an old Postal Jeep on the way to work one day for a few hundred. Ended up modifying it heavily (V8, chop, sectioned, lowered, etc.) and supposedly it spent time in the Petersen Automotive Museum at some point, after the buyer had bodywork and paint done to it. I think I had less than $600 in that one.
Well, you get the idea. It was a little of everything.