People go to craigslist to find bargains so that puts an upper limit on the price you can expect to get for a bike. To successfully flip a bike, you need to find something that's more toward the middle of the pack in terms of quality and for whatever reason the seller is willing to sell pretty cheap. It could be because the bike is unrideable because of some minor issue. A little effort (like washing it) and they could ask for more money but some people don't want to deal with it.

When I was more interested in spending time fixing bikes, they used to just find me somehow. A surprising number of people have non functioning or even working bikes that they just want taken off their hands. Folks used to just give me bikes. I'd spend $50 to $100 on them and sell them for for $125 to $175. Not a great way to make money considering the time involved but at that point in my life I enjoyed it.

It's harder to do with road bikes than it used to be. The world has figured out that there's a demand for them and they'll want $200 for any piece of junk with drop bars.

I must confess that I myself am guilty of a fixie conversion. It wasn't a high end bike though and the fork had been destroyed in a crash. Plus I left the derailleur hanger in place along with all the cable guides. I also saved all the components (which was pretty decent shimano 600 stuff) so it would be easy to convert back into a geared bike if I wanted.