I'm really not sure how ethics comes into play at all in this scenario. Buyer sets a price, I pay it. It is not my responsibility to educate the seller, it is my responsibility to complete a deal as agreed upon, in this case paying what THEY asked for.
For a well known item like this, even someone with zero knowledge of what a pinball is could get at least a rough ballpark of price almost instantly using google. No special knowledge needed what-so-ever. That doesn't really apply to the 15th century vase example as that is not a common item which can be recognized without some level of special knowledge. It isn't literally spelled out on the front of the item the way "The Addams Family" is.
There may be an argument to be made of someone not in the hobby selling a TAF gold for a standard price as that would be an easy mistake when researching, but no matter what they should know the market value is well over $100.
Again...this scenario completely changes if they ask me what a fair price would be. If that happens and I tell them $100, that would be unethical. But since they used whatever method they used and came up with an asking price without my input, that is not my responsibility in any way.
This situation has absolutely nothing to do with people offering / accepting more money after a deal has been agreed upon with someone else. That is 100% a dick move all around, but is completely different from what is being discussed here.