I'm sorry, I looked but didn't find your /sarc tag. I can only assume you overlooked it while posting as you can't possibly be serious.
I am being very serious. The man who posted the 'ads' is primarily responsible, but any site that makes a single dime posting such ads (even through side advertising) has a responsibility to vet those ads. Again, I ask, if the owner's of Craig's list didn't know this was an invalid ad, how could individuals visiting the site be held responsible for not knowing?