You got to ask yourself what you want the thing for. As has already been pointed out, the 10-5 is too common, so that, unless it is new or as new with all papers and box, this won't ever be a rare collectible before you die, and since this apparently isn't NIB, you'd be getting yourself another shooter.

So if that's what you want, point out to them how long it's been there, that they were willing to give it to you for 400, and the degrading that's happened since then, and tell them you'll buy it for 350. Then you'd be in very good territory price-wise.