When you or a friend show up to a public place to buy it, it’s usually pretty clear who’s selling it.

Tried that but I guess they got suspicious and didn't show up, or sold it to someone else before me.

PSA for anyone with a bike: register it on https://bikeindex.org/. I registered mine there the day it was stolen and almost immediately got emails from several people who were monitoring a known bike thief's Marketplace account and saw mine listed.