Fountain Pens are not regulated and widely traded commodities. One cannot simply look up the latest price in a newspaper, and count on  an outside agency to ensure the provision of correct product.  Collecting old pens is a hobby, an engrossing and entertaining one for those of us who have found our way to it. But it can be intimidating to make purchases when one lacks experience.  This can generate a "catch-22" in which one is  fearful of buying vintage pens without having experience, but indeed needs to buy/sell  such pens to gain that  experience.

How to learn? Visit a vintage pen shop. Read Books on the subject. Go to a vintage penshow- the most valuable resource to the neophyte pen collector. Find a mentor. Ask questions. Don't assume. Listen to those who truly know. Peek at the online pen groups and websites.

Ebay is a tricky place. Bargains happen. Horrible purchases happen, too. Finding a pen on the 'bay for slightly lower cost than at a retail location might be no bargain.  Many honest amateurs do not know what constitutes an  "excellent" condition fountain pen or what constitutes a "rare" pen.  To many, if the pen is not broken in half, it is "excellent".  True fraud occurs sometimes, as well.

The root of grading disagreement stems, really, from the use of grade to convey value. It might not matter whether  "moderate brassing" indicates that a pen grades Excellent or just Very Good , were in not for the impact that difference has on price. So, to be blunt, a big part of grading has to do with pricing. One should seek accurately graded pens so as to pay proper prices for pens and to have the chance one day to sell at proper prices.

Ultimately, the "right price for the pen" is the key point to identify, since in the retail arena grading is being used to convey value.  To an expert, independent of labels,  a $100 pen is a $100 pen (a somewhat recursive reference) whether one then labels that pen as VG or as Near  Mint. Whether we apply a conservative "VG", an overly optimistic "near mint" or a painfully qualified "Excellent++ except for moderate brassing, tooth marks, and weak imprint", the trick is for the pen to end up at the proper price.  The risk, as with all vintage collectables hobbies, is to encounter overgrading with associated overpricing.  Of course, dealing with that can take a lifetime to learn :-)

Keep in mind that a Full Excellent vintage pen should have clean trim with (arguably) only trace high point brassing (smidge on a clip ball, etc), good imprint, solid threads, clean nib (though some grade nib as a separate piece), etc. Also, keep in mind that MOST PENS are NOT up to the Excellent grade. Various minor flaws do lower the grade, and should lower the price- but such pens are still very collectible.  The trick is to pay the right price for the right pen. The trick is not to assume that because a pen costs a few dollars less at e-auction than in a reliable vintage pen-dealer's store, then it must be a much better deal -though it might be :-)

Most guides to old pens  give prices based on "excellent" condition.  But Excellent means more than "hmm- looks like everything is there".  In general, pens in Very Good condition are worth only 70% of excellent, and nearmint-to-mint pens can carry a premium of 25%-100%, or more, over excellent.  And, I would suggest that most ebay pens are far, far from excellent, once one realizes what excellent means.  And other issues, such as Color or Originality of Parts, which are not grading issues, per se, also can greatly affect value. To be prepared to buy pens well at a penshow, at a store, or on ebay, one must know how to grade and how to ask questions which pertain to grading.
 

When one is prepared and skilled at assessing  both the features of pens and the problems which arise with those features, he will be better prepared to have a positive and productive experience with this hobby.