Greek coins could be archaic, classic or Hellenistic. You can see why a collector who is a recognized expert in one field will be eager to learn in many other areas. Yet you meet dealers who can't even identify the coins they sell. Dealers who don't know their coins assign wide dates to them. Nationally-recognized dealers in moderns will often drop the ball when it comes to ancients.
Ancients take a lot of work and knowledge.
One way to judge a seller is by their library of books and catalogs. Books are expensive and we'd all rather own the coin than the book, but if you want to know your business, you practice your trade, unless you are just in it for the fast buck. We never clean modern coins. However, after 2000 years in the ground, ancients often need cleaning. But soap and water or even an industrial degreaser won't do to a silver coin what a buffing wheel or a night in Dip will.