I started flipping bikes when I was in college after I restored my first vintage road bike. I found that all kinds of bicycles were readily available at local thrift stores in the college town where I lived. I’d often find bikes from $8-20 that I could flip the next day with minimal cleanup and tuning for $100 or more. Even found some nicer vintage bikes priced cheap because apparently nobody knew what they were worth. It was trial and error at first, but with experience you learn what things are worth, what to avoid, what to jump on. The landlady was nice and didn't say anything about the pile up of bikes and parts on her back patio.
Now that I live in Portland, OR, I can’t find a bike at a thrift store to save my life. I've flipped a few in the five years I've lived here if I just happen to bump into something good at a thrift store or garage sale, but that almost never happens.
The most important lesson I learned when I did it actively was this: if it became a job rather than a hobby, I totally lost interest in it. Now I do it when I can because it’s fun and because I can make a little extra cash to spend on my hobby. I think if I ever tried to make a business out of it, I’d lost interest. That’s why I don’t do any spreadsheets or anything- not that there’s anything wrong with that, that’s just how I roll.