If you look on eBay at what machines like that sell for, you'll get an idea what it's worth. I paid quite a bit less than half for an older one in a box like that, with two manuals and a full set of attachments, but it had a replacement footpedal. And that was an auction, and there were several other bidders.
My personal opinion- the seller is way off track calling it "antique" or "museum quality". 1. it's not old enough to be an antique, and 2. it's way more worn than a what I'd want it I was looking for something for a Museum - I've seen very nice ones and this one is not even close.
here is a much nicer one that sold recently
http://www.ebay.com/itm/WILLCOX-GIBB...p2047675.l2557