I flip quite a few bikes as a hobby and for a little additional income, I buy them as cheaply as I can and completely go through them before selling them.
My son found my first project a 1977 Raleigh Super Grand Prix with a free sign on it in our neighborhood and it was just my size and after the fun I had building it I was hooked.
I do not do fixie or single speed conversions, I just rebuild them to the best riding condition I can and sell them at reasonable prices. I haven't found much in the way of high end stuff but I find a great deal of satisfaction in rescuing a bike from near certain doom and giving it a new life with someone that wil hopefully ride it and care for it in the future.
I'm not terribly fussy, I'll fix up anything I think I can make money on but I do tend to stick to old road bikes. I just don't have much interest in working on anything else.
I've built and kept a couple for myself and for other family members as well which is also fun, the whole family is on vintage road bikes now.
The guys that drive me nuts locally are the ones that grab up every dirt cheap bike they can and try to flip them without doing any work on them which drives up the prices on a lot of other bikes that should sell for a lot less than they do.