It would be interesting to see examples of the alternatives. That is, marketplaces that failed due to scammers, marketplaces that succeeded by heavily curating to block scammers, and marketplaces that failed due to excessive curation while trying to stop scammers.

I wonder if scamming is actually a symptom of a successful marketplace. Scammers presumably have to think through their opportunity costs and would want to focus on marketplaces with many buyers and sellers and high transaction volume.