I flipped bikes for a couple of years. I got started because there was a decent metal scrap yard in my town that had a pile of bikes to pick through. People left a lot of good bikes out on the curb in that town on trash day, too.
We moved to another town with a decent recycling scene and plenty of good yard sales. Somehow I ended up with a pile of British 3 speeds. I'd buy them for 0-$25. go through them completely, wash, wax, new grease in all the bearings and decent tires (from one of the other bikes in the pile) and sell them for $100-$150. Decent money for 3-4 hours work and I could stand behind what I sold.
No real reason for the 3 speed thing other than luck, taste and maybe a little Sheldon influence. They were selling well in Boston, too. I'd get people driving all the way up to get a bike. Finally, there was this guy who'd stop by about once a month to shoot the breeze and he'd take a couple of bikes back to Boston when he left. I'm sure he made money on them, too.
I dabbled in other bikes. Japanese road bikes were worth the time. MTBs didn't seem to turn a profit, nor did kids bikes but I did the kids bikes anyway and gave them to the neighbors. 50s ballooners made money but I never seemed to have the right bearing cone or skip tooth master link in my pile of British 3 speeds.
We moved to a smaller place and I don't have the space now, nor are the recycling days as good around here so I just work on what's left. I'm down to 15-20 bikes. The goal is 3 bikes but I'll need a little time to let some of these babies go.
I was up to 50 at one point. I probably sold 50 over 2 years. The rest I put back in the recycling bin after stripping them down. I probably gave away 20. Say a hundred bikes total. 30 being Raleigh 3 speeds.