Case wear on tank De Ville a cause for concern?

You don’t really provide enough information for us to do anything but guess. But there are some possibilities. One being that it is a yellow karat gold case, and somebody preferred a white colour, so had the case rhodium plated. The answer there would be either to re-plate the case, or reverse plate it to remove the rhodium, then either rhodium plate it again, or wear it yellow. Second possibility that the case might be brass, and the plating has worn off. The brass may have been polished to make the watch appear more presentable. The answer there would be to have the case stripped, and re-plated either white or yellow, which ever is your preference. In either instance, a case for concern? Yes! Unless you buy this with your eyes fully open about what this case is made of, you will face the expense of restoring the finish, and hoping you like the result. Then having to repeat the process in the future. I suggest don’t buy it!

So this is yellow gold plated over a white base metal? If you buy it, the price should be next to nothing IMO, since I am guessing that the seller won't get many offers.

I was thinking rhodium plated over a yellow metal with the rhodium wearing off. But it could be yellow on white, I suppose. With yellow becoming exposed at the points of wear, it points to me to a yellow base metal. But until I knew what the yellow metal was, I wouldn’t offer beans for it!