There is a guy on Facebook called 'Rob Bear' asking anyone who sees an advert for a mini on eBay, Facebook, gumtree etc that they think is either a ringer or illegal to message him and he will look in to it. He states he's a police officer and best placed to look into it so not to spook the seller.
He's a member of fb group 'mini classic 1959-2000' and also the stolen classic mini page.
Not sure if he's a member on here as well, but it maybe worth letting him know when you see anything a little dodgy.