My take on this is that ASSUMING you can find something worth finding for $25-50, you probably need to put an additional $20-40 in consumables into it, and a minimum of a couple hours of cleanup and tuneup work, plus another hour or more on the sales side to have something that you MIGHT be able to sell for $150-200. That's quite a bit of labor and risk to MAYBE make $70-100.

A more realistic way to approach flipping would be to skip all the low end stuff, and find GOOD stuff that might be under-valued at $100-200 because it's old, but properly tuned and marketed to knowledgeable buyers, might command $300-400.

The cheap finds he's talking about are better served as gifts to kids or donations to less fortunate folks who really need a bike for basic transportation. That way, you'll get a lot more out of it in Karma and or good feelings about yourself than you ever really could in profit. Or maybe keep one of them for yourself to use as a bad weather/high crime zone bike.