Morgan,
Excellent point. I have yet to find a rating system or guide book for the pricing of vintage vacuums.
You can however, visit Kelly Blue Book and NADA to find values of vintage autos and current pricing as well. I'm sure there are other sites that provide pricing guides to other collectibles.
Vintage vacuum cleaners are probably never going to have it that cut and dried. Placing a value on them is sort of a guessing game. When Stan Kahn had his Hoover 950 on ebay, the opening bid, I think, was $600.00, (and it had a reserve as well), or therabouts. The auction ended early as someone offered what Stan wanted, (amount unknown), and then the auction was ended early.
Does that mean every 950 will get that much money? Probably not. However, it does set a precident, realistic or not. And, we all know that are professional ebay selling sites and the owners watch all these sales carefully. That is why a totally mismatched Kirby 560, with mismatched accessories will start with an opening bid of $99.99 and someone will buy it!
Then anyone of us can go to an estate sale and find a Kirby 560 in mint condition, rarely used, in the original box, with all the original accessories and buy it for $15.00.
It reminds me of car ads I see in the Antique Auto Trader. Some fool will have a 1976 Cadillac Sedan DeVille, with a grainy picture, at the wrong angle, 2nd owner, non-garaged car from the rust belt with 62,000 original miles and have the price listed at $12,000. Where in the hell does the seller come up with a number like that? Is the seller just picking a number out of the sky? Yet, all it takes is one buyer to come along and presto.........is that now the value of all 1976 Sedan DeVille's with similar criteria? Hopefully not.