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. If a silver coin looks too shiny, it was abused.
Cost is another reason that people find bad bargains in ancient coins.
They are by no means rare. With this coin, the key is not its mere grade, though grade counts, but its centering and position. Cheap Owls are crowded, with no extra metal around Athena. A lot of them have test cuts. If you aren't actively collecting ancients, don't buy a coin that isn't attributed.