I own a Certo 6. It is a 6X6 folder which was made in East Germany. It is one of the very few folders that uses a standard film advance lever (two stroke). The lens is a f/2.8 80mm Tessar which is very good. They are fairly heavy for a folder though. Even so, I wouldn't mind owning another one.
I'm with Chrismat, it's a great folder. I owned ones years ago and really regretted having sold it, so I bought another from Jurgen as a Christmas present to myself this past winter. It goes with me everywhere now. I would love another at some point, but can't justify it at the moment.
You’re right, of course — it just didn’t occur to me that he’d be selling a Certo 6 that he’d purchased from Certo 6! He referred to Certo 6 as the seller in his short ad, so I took his ad subject line “Certo 6” to refer to the well-known seller.