I don't know whether there are many SKB collectors out there, but if there are, that gun would have some value as a collectible. The 150 was only made for 3 years, and you don't see many of them compared to the 100, 200, or 280 models. It has a BT forend and engraved receiver like the 200, but is an extractor gun like the 100.
There will always be a premium for new in the box. Chances are a not very used SKB will be pretty much problem free, but since most inspections don't include shooting privileges, you don't know for sure. With one new in the box, it only has whatever problems came with it from the factory. And even keeping the box after it's used makes it worth a bit more.
I agree with Don. $1500 is about new Uggie money. If I were looking for a 20ga in that price range, I'd be interested. Probably try to get the seller down some, but I don't think it's an unreasonable asking price.