These guys are putting you on.
That gun is worth $100. However, since I'm an incredibly generous (as well as honest) person, I'll send you a check for $150 and take it off your hands.
Just kidding.
Seriously, I'll bet my wife would give you $450 for it. She shot more than 200 rounds out of our Model 15-2 (1963) last Sunday. She was grinning from ear to ear, having the time of her life. She even let me shoot it a few times!
I gotta admit, the 15 is one of the best damn guns ever produced.
We paid $350 last year for the one shown. It's worth more, especially with the box. I'd give at least $400.