I bought my M340i Touring from a specialist performance car dealer in Hertfordshire. I'm in Central Scotland. Painless experience from start to finish, had no issues at all.
I was wary of buying unseen but I wanted a specific spec/colour/mileage G21 and they were simply unavailable where I am. Sales guy I dealt with was top notch, sent a link to a 4k walk round video and loads of pictures.
I had it delivered to me and the car was everything I expected. The biggest worry I had was bank transferring the £47k payment to someone I'd never met before, was a bit of a bit of a nail biter!!
I'd buy from distance again but I guess I had a positive experience, there's no cast iron guarantee that the next time I'll have the same, so yeah - YMMV.