I've been flipping bikes for years. I love working on bikes and only work part time, so it works out for me. It allows me to get some grocery money or whatnot but sales can be hot/cold. When I first started back in 2003 I was making quite a bit more money on each bike. Mind, it was very different in the early 2000's. I was doing fixed and SS conversions (mostly) and there was a very high demand back then as only a couple of Manufacturers were selling complete fixed bikes and folks were tripping over themselves to buy conversions. No one sold separate wheelsets* and other parts* like they do today, and with everyone selling fixed wheelsets/parks and super cheap bikes now, it's almost impossible to make much money off of each bike. The only way I make money today is from scoring a super cheap bike on Craigslist/Garage sale or a donation from a neighbor. The other way is to scour Craigslist for deals and act quickly. I've gotten a 1969 Schwinn Paramount P-14 complete track bike for $275, a Titanium Airborne Zeppelin w/Dura Ace groupset for $250, you get the idea. The other way to make more money is to strip the bike and sell off the parts/frame/etc. Again, Craigslist can be kinda fickle, so I'll put stuff up there for a week or two then off to eBay it goes.
And I also strip all my bikes down to the frame and pull the fork. Ground up rebuild. I may not make a lot of money on the bikes but it makes me feel good that something of quality is leaving my shop to another. I also take throwaways that are low end and try to save them. Any bike is a good bike if it is of some use to another. I had a crap Huffy fixed conversion that was my grocery bike for about 4 years. I finally sold it to a substitute school teacher who fell in love with it. I think I sold it for $20 because it was so low end. That was 5 years ago and he still brings it to my shop when he needs a part replaced. I am tickled that this damn thing is still rolling. I fix it and parts for free for him because he loves the bike. Passion for his bike, a cheapie still rolling and not another casualty for the landfill. What more could you ask for?
*fixed