Remember, that
ebay listing is merely suggesting a "Buy It Now Price". This same approach is used for any number of items on
ebay (not just stamps).
As stated earlier, someone who may really be interested in paying a premium for the item could easily offer a fraction of that "Buy It Now" price and still win it at a much lower cost.
It comes down to do you want the item bad enough to "Buy It Now" and not go through the auction process which allows it to go to the highest bidder.
As for the stamp in question, I could be a millionaire and not have any interest in purchasing that particular stamp, superb or not. I have copies that may not be as "superb" but still more than adequate for my collection. The stamp isn't particularly rare, either.
BTW, a MNH Scott 701 in "superb" condition is currently listed in the 2011 Scott Specialized at $475. Not sure if the quoted value by Russ suggesting that SMQ is now valuing it at $550 may be a later reference, or is that is the value now referenced in the 2012 catalog, in which case the stamp has appreciated about 16% in a year. However, it would need to appreciate at that rate for the next 20 years to come close to a 320% increase in value, which is what the
ebay seller is asking for the item.