A private sale used bike is worth whatever a seller and buyer can agree on.

The ONLY things that matters are how badly you want the bike and how badly the seller would rather have the money than the bike.
Regardless what the Blue Book says if you are a seller you have to find a buyer who agrees that price and, as a buyer, you have to find a seller who agrees. You can't force a seller to accept a Bicycle Blue Book price. The Bicycle Blue Book may give you a talking point but, since the seller obviously isn't using the bike, I'd rather talk about money in his pocket vs. something that he doesn't use taking up space.