If you're negotiating to buy something–a house, a company, even as the purchasing agent for a big company–you're also selling.
That's because it isn't a faceless transaction involving a list price and a credit card. The purchase involves faith and trust and risk and someone caring enough about the other person to do more than seek the highest possible/lowest possible price.
If you hope to buy for less than the clearing price, or get better work than average, in fact, you are selling when you're buying.
A friend was selling his house, and every time he showed one particular prospective buyer a new feature, the buyer discounted it, demeaning its value. This is a common strategy–denigrate the thing you are hoping to buy, question the judgment of the seller, make them feel desperate. After all, the thinking goes, a desperate person will sell for less.
Perhaps. But more often, a person being made to feel desperate will take her valuable goods somewhere else, to someone who cares more.
Why not say, "that's fabulous! It makes the house worth even more, well done." Recognizing the good work of those you hope to buy from puts you on precisely the same side of the table as the seller.
The brutal purchasing manager who uses RFPs like a club and nickels and dimes (not to mention dollars) suppliers–do you think he's actually getting their best work? When we're talking about emotional labor, it's not often about the money, it's about how the transaction makes the seller feel.
Or consider the used car dealer. His business is only as good as his inventory, of course. So where to get the cars that get sold? One strategy is to only buy cars from desperate folks, folks with no options. (Or to make the people you meet with feel desperate). What sort of inventory does that leave you with? Last resort cars from people with no options. When the dealer who sold me a car recently tried this tactic with the car I hope to sell, I walked away.
The good stuff is more likely to be sold to people who care. The things you'd like to buy are probably going to be sold by people who have other options now.