Yes, this is the genuine camera used by Oscar Barnack to photograph the Loch Ness monster. It's worth approximately $1,000,000,000,000.
Signed,
David H. Bebbington
Photographic Expert At Large
PS: The mounting of the camera is the clue. Imagine the camera pointing downwards with the big metal clamp attached to a heavy-duty laboratory stand. It's a microscope or macro camera, probably pre-World War I. It's not especially rare, they made quite a lot of them, or at least sold them under the Leitz name to go with a Leitz lens, of course not too many survive after 100 years or so. The bellows of this example is of course shot, and I can't see any glass in the lens, also the pneumatic shutter may be having a little lung trouble (seals on its tiny cylinders). On the other hand, the camera may go cheap and with a little effort make a nice display piece - it wouldn't be too hard to take a picture with it, either, with new bellows and a working lens/shutter.
PPS: The seller is not correct in saying there are no mentions of this camera in literature. There's a picture of essentially the same camera on page 13 of Rogliatti's "Leica The First 70 Years." These cameras were produced from the 1880s onwards, so it looks as if this camera, which may well sell cheaply, comes with full bragging rights about owning the oldest and rarest Leitz (not Leica) camera in the world!