You are right. That is not addressed directly by my statements. At a good auction it becomes a delicate balance of encouraging bidders to bid to the limit where they themselves set the value, discouraging shilling, collusion, and other cheating, and maximizing revenues for the auctioneer (and/or property owner). That actually takes a lot of skill, effort, and knowledge on behalf of the auctioneer. There are several types of auctions, and I don't want to try to describe all the differences here, so I'll confine it to several of the larger on-line auctions. Some formats do not 'protect' the seller, as an example an 'absolute auction' where items go to the highest bidder, and the bidding starts well below any estimated value (often called a 'penny auction'). Some auctions protect the seller by placing a 'reserve' on an item, or a limit that must be met or exceeded before the item is declared sold. Some auctions start at the price the seller wants to realize. And some are not auctions, but rather an offer to sell at a stated price. In the latter three instances the seller does not necessarily agree to sell to the highest bidder.
There is an element of faith needed by the seller for the 'absolute' auction. To maximize faith, the selection of a good auctioneer is essential. A good auctioneer on the big sites generally has a following of clientele and provides exceptional descriptions including many high-res pictures. They know what they have and present it well. Several sellers come to mind - Woodlawn Boys, Jack-the-Dog, Sarah, Bryant Ridge, and there are many more. Some are better than others in certain ways, but my observation is that when they do an absolute auction, the price realized is most often well beyond my bidding limit.
So back to your point; yes, often the seller agrees to sell at whatever price the high bidder offers, regardless of what it is. And if executed well by a 'professional', the hammer price usually matches or exceeds the seller's expectations. It takes some faith to buy into that concept, but a bit of faith (and a competent agent/auctioneer) can yield some interesting returns. Those without the open arms of faith can choose an auction with a reserve, a high opening bid, or a set sales price. From personal experience, starting an absolute auction with a very low entry bid can be disturbing, but if well done, quite rewarding.
So, it is a different atmosphere striking a deal with an individual or at a gun show. It is usually a sort of reverse where a price is advertised and the buyers haggle for a lower price. And pricing by a seller could be way off either way, high or low.
It's all supply and demand no matter how it is sliced or diced, just different ways of finding that sweet spot where a potential buyer reaches in the pocket and pulls out the cash.
Sorry to have gone on so long, but it is a complex subject when you get into the weeds.

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