I agree with Charles....
Unless its a very rare antique, most dealers wouldn't even give it a passing glance, especially a vacuum cleaner from the late 1970's that MANY are still in existance today; in fact I counted either eight or ten for sale on eBay just this morning. You have an advantage over most of the sellers simply because your machine is unused in the box, but my suggestion would be to let the bidders determine the worth of the vacuum. Not that long ago I watched a model 1205 that was brand new in the box with the lid cut open like yours sell for over $900. By the same token I have a 1205 brand new in the box that I got for a little over $100, so it really depends on how bad the bidders want it. If you have a figure in your head, then set a reserve price on it, that way it won't sell unless the reserve is met, and anything over the reserve, well that's just extra profit for you.
Just something to think about, but when you do decide to list it, let us know! Thanks!