I have bought and sold cars my entire adult life, often fixer-uppers and project cars. If I had to put a $/hr guess on it, I'd bet I made.. maybe a dollar per hour on average? :-) Kidding, but not too much.
My problem, and I suspect many other "car guys" problem, is not treating it like a business. We buy cars we're interested in, try to repair and upgrade them to a level that we consider attractive, and then when we sell, it's to a limited market that may not want a car modified in just the way we do.
My profitable flips have been utilitarian cars that I had no interest in restoring or hot rodding, that I just made driveable and eventually sold. My last flip was a mid 90s Ford Thunderbird with a blown head gasket but a good body and interior. Bought it from a co-worker for $300, fixed it, and sold it a couple of months later for $1800.
The current occupant of my garage is a 87 Buick Grand National, bought for $2k with the worst pimptastic candy aple paint job you've ever seen, every mechanical component under the hood rigged (it had an gutted turbocharger with the oil feed crimped off!), etc.. I have it running with a new motor and have put about $1k in additional parts into it, but it'll need much more work to be worth anything approaching a profit. I like the car but it would've been a lousy business proposition.