I certainly don't think Roy did anything wrong here, but man...this discussion is taking me further down the path of thinking that the "best" approach in selling a book is just to list the book and the asking price, with no reference whatsoever to any market data, price guide values, etc...
In the comic collectibles market, there will always be information asymmetry that cannot be fully resolved. If you don't want to pay my asking price, give me your offer and the data/rationale that support your offer, and I'll tell you what data I think support my asking price. It's just basic communication/negotiation (which should be done via PM, of course)...if we think our data are too disparate, we won't do the transaction
So, wait a sec...what's the real issue here that we're concerned about? Suppose a hypothetical seller (not Roy) posts a book for sale, and cherry picks some GPA data that supports his/her asking price. Is it just irritating to us on general principle, or are we truly concerned that a naive, uninformed comic collector is going to come along in here and pay "too much" for the book? Is that why we feel the need to post up data counter-examples in peoples' sales threads? Do we have "protection of the uninformed comic collector" in mind? If so, that's a tall order.