If the seller is in a monopoly position (or indeed if the buyer is in a monopsony position), I don’t think we can talk about free trade, so this would be outside the scope of the article.
But most trading exchanges are not between single (or excessively powerful) parties, and still we consider sellers as those who gain, and buyers as those who lose.
Prices are supposed to be related to costs in order for an economy to operate efficiently, but now days, businesses want prices to relate to value. This means that consumers and workers lose
Is this a personal opinion or do you have a reference for the claim that “prices are supposed to be related to costs for an economy to operate efficiently”?
In a free economy prices are determined by the market: the supply of and demand for a good or service. Both production costs and perceived value are relevant in that one puts a floor under the price (a seller is generally going to be reluctant to sell below cost price) and the other a ceiling above it (a buyer is generally not going to pay more than the perceived value). It is unclear to me how you would determine that the efficiency of an economy would be higher at one price point between these two than at another one.