I bought my 2nd hand M8 about a year ago also from a store. I immediately noticed similar problem to yours, but worse. I had several lines.. Anyway, since it had short warranty from the local store they had it sent to Solms for repairs. Over all it took about a month and the camera came back fixed and cla'd. I also received complimentary 1 year factory warranty from Leica.
Though I admit it was pure pain to send my "new" camera away during the first day and being without it for a month, I'm very happy with the service I got. And most importantly with the camera. It's been working flawlessly ever since.
The actual repair document stated ccd pixel fault as the cause and cost without warranty would have been 320€.
//Juha