Seller responded fairly quick yesterday evening, and offered to take the camera back, with apologies. Both the ad and the email (in German) had some misspellings and I assume writing just isn't his best skill. As a result, I do not think the seller lied about the camera, just that he is inexperienced with Barnack Leicas.
Today I had a thorough look at the camera with my friend Derk and we agreed that the curtains are indeed newly fitted. The shutter and speeds look real good.
I'm thinking the camera was serviced, somebody put the pressure plate back in wrong and the RF apparently was completely discarded in the process.
Tonight I'll write the seller and ask if the camera was serviced with a warranty, and request him to have it fixed under that warranty.
For a 1932 black paint Leica II it's a real looker and half way to functioning perfectly, so I'd like to get it spot-on after all.
Thank you all for your replies and for helping me collect my thoughts on how to go about. I'll update the thread when things evolve!