There's a pretty long list. If someone sells a fraudulent product by mail, I don't hold the mail carrier accountable. If I buy a product I don't like on Craigslist, I don't hold them accountable.
The mail carrier does not advertise the fraudulent product and does not handle the financial transaction. There is a pretty obvious difference between a mail carrier and Amazon. Craigslist actually is responsible for stuff that is offered on their website, see also the long-running debates about sex workers advertising on Craigslist.