Gazumping seems unethical, but it's also a quirk of language and agreeability. The "gazundering," aspect of this scheme is interesting from a negotiations perspective. If anyone accuses you of it, I'd recommend rejecting the accusation as bullying, and always refer any bid to looking at it when it's on paper.
If you have bought a car and agreed on the price, only to find there is another 30% of "fees," insurance, financing costs, freight, and random whatevers, or taken a minimum wage job only to be charged a uniform rental fee, you have been taken by a similar scheme. There are people who do this kind of scamming full time as their living, we call them "salespeople."
If a seller starts the discussion by advertising an offer to sell for $100, and the buyer agrees to continue the conversation at that anchor price, and then after getting details about the goods, the buyer comes back with a lower offer of $50, that's a pretty transparent negotiation. Naming this "gazundering," seems like a ploy to discourage beginners from negotiating.
It is the seller's responsibility to line up other offers, and it's on them to manage the opportunity cost of any discussions. Using deception to get leverage on the seller by using a fake offer that causes the seller to let go of their other opportunities would be a more accurate description of the unethical aspect of "gazundering," but even then, high stakes negotiations can become a kind of warfare.