https://www.techdirt.com/articles/20210430/15051746714/content-moderation-case-study-craigslist-implements-phone-verification-to-fight-spam-spammers-fight-back-2008.shtml

Summary: Craigslist -- the online marketplace that pretty much still looks the way it looked when it went live all the way back in 1995 -- has the same problems every online marketplace has: spammers and scammers.

The battle against people seeking to abuse the system has been ongoing since the site's inception, but in 2008, Craigslist implemented a new control measure that temporarily stymied spammers who had found several ways to beat the systems previously employed by the online market.