I have a gun that I'm contemplating selling for $2,500. They offered me $1,750. I heard the same explanation about 70% of Fair Value because they "had to make money too", which is fine. They're not making 30% on any new gun they sell though. Also, take a look at some of those used guns on the rack. They ask way more than fair value on the few I've peaked at.
Think of it this way, if you got "fair value" in this scenario, you make almost 43% more. Even if you give somebody a deal, you're making a heck of a lot more.
Like most things in life, it's probably the right decision for some people but definitely not for all.