A tale of five coins:
Ancient Coin Grading

IN A NUTSHELL: The grading of ancient coins is no less subjective than the grading of modern coins. Even experts are ... human. Expect inconsistency. Learn to grade for yourself.
	Both Alexander III staters (both from Abydos) were graded gVF... by the same dealer
	Istros drachm graded gVF by Dealer A
	Istros drachm graded XF by Dealer B
	Istros drachm graded F+ by Dealer B
	Istros drachm graded XF+ by Dealer C
	The grading of coins, ancient as well as modern, has always and will likely always be subjective. What's more, grading with ancients isn't universally defined as meaning the same thing by all dealers and collectors. Some define it as referring to wear only, some to wear and strike (overall detail), some to overall eye appeal (called "market grading" with U.S. coins), which factors in corrosion, dings, patina, and sometimes even style. Dealers should adequately describe a coin when selling it, given the vagaries of grading. Good photos, fortunately, can go a long way to overcoming the above inadequacies, though not completely. Most ancient coin dealers use the European system for grading coins, which is more conservative than the 70-point U.S. system. An AU, according to the U.S. system, is an XF according to the European system. Unfortunately, the U.S. slabbing services, including ICG and NGC, use the same grading system for ancient coins that they use for U.S. coins, which is confusing and potentially misleading. They grade coins higher than ancient coin dealers. Some dealers stray greatly from any known grading system, particularly on eBay. You frequently see coins, for instance, graded XF+++ that are aVF at best. Here are four good sites with more detail about ancient coin grading: Doug Smith's Grading and Describing Ancient Coins Calgary Coin & Antique Grading and Quality Classical Coins' Grading Ancient Coins Brad's Grading