My bike was stolen recently, and when I posted it on BikeIndex I almost immediately received emails from several different people who had been monitoring a known bike thief's Facebook Marketplace account.

I didn't end up recovering it, but I was still impressed with how well the site worked, and I realized it hasn't been discussed on HN except for some very dismissive comments when it originally launched (https://news.ycombinator.com/item?id=6467431).

Darknet Diaries podcast episode 153 is all about Bike Index. The founder is the guest. Great stories.