The frame is/was a black finish, the slide was steel (not sure if it was polished stainless or a matt finish.
It should have a matte bead-blast blue finish.
RUN, don't walk, back to that shop!
Also, FYI, any S&W 59-series mag should work in this pistol, along with the (admittedly uncommon) 20rd extended mags made for the 9mm Marlin Camp Carbine. Although the recent run on >10rd mags has cleaned out most new magazines from the big sellers, I noticed at a recent big gun show that 59-series mags COULD be found if you're willing to sift through the stacks of random used and often unlabeled mags at the individual vendor tables. (I was looking for mags for another pistol and happened to notice numerous 59-series mags while I was doing it.) IOW you CAN find mags for this pistol if you are willing to hunt for them.