I've had a couple folks come in and ask for appraisals. I'm happy to do it (you'd better be able to come up with ideas of value when you're buying and selling), but I also have to be honest - it's an incredibly science-less science. When I'm selling something for $500 that someone on 1stdibs is selling for $4000, there's more than a little play. When I see a pair of chairs selling at one place for more than a set of six is at another, there's a lack of consensus on value.
Why does this happen? I think there are a few reasons. First, there's really very little information available to demonstrate scarcity, or lack of it. Production runs are unknown, as are surviving quantities. The best we can do is ask "how often do we see these come up for sale?"
Second, people have differing opinions on condition issues. Does refinishing help or hurt? Is a small gouge a big problem? There doesn't seem to be strong consensus.
Third, and this is the one I can least account for, is audience and knowledge. The auction audience has little time for Knoll sofas or Paul McCobb Planner Group products. These were mass-produced and are visibly non-scarce. Nonetheless, the general consumer market seems to be willing to pay quite a bit more for these things. So should I appraise based on the possibility of an enthusiast's impulse, or based on the cool eye of the sophisticated buyer? The unlimited wallet of the SiliValley interior design client (which really can warp prices), or the more cautious mid-century fan?
The internet has also led to a breakdown of traditional ideas of retail, auction, and private party sales. Everyone on craigslist and eBay thinks of themselves as retailers, who should receive top-dollar for their items. This creates further confusion about pricing. If a price is visible on eBay, maybe that represents something. That eBay seller may sit on the item for years, hoping for that magical sale that never comes, but the unrealistic prices asked by some folks are nonetheless data points that have to be considered.
In short, appraisal is inexact, a bit chaotic, and you can get a lot of different numbers on the same thing.