Filtering the timewasters and tyre kickers when selling a site - how do you do it?

I've got a couple of live listings in Flippa and am discovering just how poor the quality of some of their members is. On the first listing I got over 40 responses many from people with zero buying history most of whom have obviously not even bothered to read the information provided in the listing. They think they come across as pretty smart, clued up potential buyers if they fire off a PM saying, "please provide full details and add me to Google Analytics" (even when I've disclosed there's no GA on the site! And that they need to sign an NDA first.)

But ... how do you filter the potentially serious buyers from those just curious to get free traffic and earning stats on numerous sites so they can decide which ones to create copycat versions of?