What should I do?

Well IF its rusted beyond use and paid $500 then I would say "misrepresented ". If you have been in contact MORE THAN ONCE over a 1 week period and still continuing to sell then I would send 1 more pm and 1 more day. Send a mod a pm and maybe flag the person in their other ads?
That being said there is 2 sides to the story. Also can you post pics of $500 parts.