I'm with you on secondhand sales being a poor correlate for actual ownership. It's completely reasonable that more for-sale listings indicate cities or places where people don't want their fixie anymore. Additionally, Priceonomics only looks at sales taking place on eBay, Craigslist, etc. So, not only are the numbers skewed toward areas that have more active internet users, the buyer may not even be in the same region.

This is why data shows fixies selling so often in Manhattan: it's all the hipsters that moved to Manhattan, who found less use for their bike, and then sold it back to somebody else in Brooklyn.

My first thought was that more used fixies for sale might mean the trend was over in those cities, and the hipsters were all trying to get rid of theirs so they could buy glow-in-the-dark parkour shoes (I just made that up).