If you haven't already, have a read through the "for sale" adverts on this site to get a feel for the level of detail that those who have a good car for sale are prepared to disclose (photos and description) and the detailed questions that potential buyers ask.
Looking at the advert that you linked to above, it doesn't contain much detail and includes a lot of the default carsales sentences. Also the car doesn't have a RWC which may not be a big deal if you sre a mechanic and know what to look for.
Whether $18k is too good to be true, is a difficult question to answer as there isn't - in my opinion - enough detail about the car to assess its value and investment required to maintain it in the short and longer run.