I'm always suprised at what some people think they can pull. Maybe it's just me but I'm always paranoid when selling a game and probably put the game down more than I should.
One thing I always try to be clear about is how many years I have owned the game (5 years is typical for me), what I have done to the game and always always state that I have no knowledge of its history before then.
The only time I would ever be comfortable calling a game HUO is if I had purchased it myself NIB and I had not routed it.
Also, unless you got an amazing steal if a deal or bought it many many years ago games sell for less than what you bought it for most of the time (at least after all the love I tend to give my games)... get over it... thats the cost of the hobby.