I don't flip much, but...I live in a small city surrounded by a relatively poor rural area, where it's sometimes possible to find old bikes very cheaply (usually out of somebody's barn). Not much demand here. Occasionally, I'll buy a bike here, put a little bit of work to it, and sell it in the nearby hipster-infested "cool" city, where demand is high. It usually gives me enough profit to pay for a fun weekend in said "cool" city. I'm pretty sure I'm making people happy on both ends.