Some sellers use Best Offer as a ploy to raise their profile in Search (although there is no reason to believe this works) some have been opted in without knowing it.
And eBay no longer discourages buyers from making Offers even if the seller has no interest in a lower price.
Then there is postage. While you give an extreme example, I have Free Shipping on some items where the built-in shipping cost is higher than the money I would accept for the item at a garage sale. Throw in fees and yes, the seller might turn down an offer of 95% of the asking price.
It's possible to ignore the automatic Accept button and only includie an automatic rejection on lowball offers.
But the seller gets to choose what she considers to be lowball. The buyer gets to choose whether to buy or not.
It's business, not personal.