I agree with Jack, every seller lists his older gun at at least 80-85%, but when you take a gun for a trade they will tell you that it is 60-70%. How do you agrue with them that their gun in not the percentage they state.
I believe the Blue Book is a good reference for condition for your benefit. The prices seem to be for the buyer, the sellers seem to use prices of what they want.
King Brown had the best answer, this board.

David