"Cleaning mark under the rear element" does not fill me with confidence, especially since there are no completely unobscured, wide-open, straight-through-the-lens shots for us to judge by.
I'll pass on this one. It's a great price, but even if it's perfectly good optically, I already have a 55mm f/1.2 Rikenon-P that's functionally the same on all my Pentax K mount cameras (and my one Ricoh, which isn't modern enough to use the P feature).
Now if it were an A lens, and I already owned a K-1... then it might be another matter. But I can afford to wait.