The rule flippers exist by is the ratio of risk/reward. Stick to the low end and buy bikes that sell or $200/under and the best you'll typically do is _far less than the minimum wage. Probably $4/hour. Every step up in margin *might* glean a better profit (but remember that we're still talking like $10/hour) at the risk of getting stuck with a $500 'investment' that's unsalable for any number of reasons.
If you look at the practice dispassionately, you'll find that flippers are typically doing the world a great service for very little reward, saving graceful bikes from tragic fate, basically for the love of the bikes themselves. Only a wide-eyed optimist could do it as a business.