Craigslist has a big spam problem to deal with; anonymous e-mail based accounts make spamming very easy. They've dealt with this through phone verification -- if you try to post to a high-spam category, or your account is otherwise suspicious looking, you get prompted to enter a phone number and take an automated call or SMS with a code to put in the site. That phone number gets tied to your e-mail and can't be used to authenticate another, so spamming now requires throwaway phone numbers (harder to acquire) instead of throwaway emails.

I haven't made any attempt to test it, but they probably block using VOIP and other non-fixed phone numbers too. There are APIs available to check whether a number is one of those types, so, say, you can't sign up for a bunch of numbers through Twilio and use those to spam Craigslist.