With no dent, it looks good. I would inspect for a deep scratch that got through to the steel under the ceramic coating, but my guess is just what Jeff said and that it is just rust from something steel or iron that set on top of the grill for a long time. Chances are it will come right off with no ill effect to the blue ceramic. But, also as Jeff said, act really concerned about it and use it to your advantage in getting a good deal. After all, it could wind up being permanent in the end.
How much is he asking for it?
Good luck and keep us posted.