You don't, yet when you go to explain how it could be used you include two other signals (came from Twitter, posts many items more than once) in order to make it work. I had originally said "I would say this method would be too sloppy to use by itself. It would need to be a small part of a larger set of signals."
So alright, we aren't on the same page, but you're saying exactly what I just said.
Breaking captcha is easier. Deathbycaptcha, decaptcher - Two of the services that provide clean APIs for the bots and breaks the captchas for you. And on the other end there are even human sitting and cracking the captchas.
It doesn't cost much and for the $2k he earns per day, these captcha breaking service is just drops in the ocean.