well- the ad says the ding is 'cosmetic'. I would certainly take that to mean that it does not affect the operation of the camera at all. Clearly- the seller is now bound to guarantee that's the case, and would refund your money were it not the case, right?
I had an FM2n with a comparable ding: It fell 1.5m on solid marble but was still usable (which saved the end of my back). I would not buy a dead cat from someone calling such a ding "cosmetic" as it is a clear sign of a severe impact which he tries to hide from his potentials buyers.