Save a picture of the item in question. A good overview pic works best, not a photo of a detail on the piece.
Go to the Google homepage, and click 'Images' at the top of the page.
Click the camera icon, at the right hand edge of the search text bar. This function searches for other instances of an identical image on the net.
Upload the picture you saved from the advert you suspect to be dubious.
Scan through the results. Look at the addresses where the picture has been found. You might find that there are more adverts for the same item elsewhere, but what you are really looking for is a dealer site selling the item, or a thread created by the owner in a forum somewhere. In this instance I found exactly that when searching for this SS helmet. It is currently for sale on raskantik for 3500 euros, which was the original source of the photos.