This tactic functions as a signal in the deal, intimating it is not complete. This places doubt in the offeror’s head and makes them see the deal in a new perspective.
This is most commonly phrased as, “Are you sure that is your complete offer?” or more bluntly, “You can do better than that!” or “Are you sure that is all?” More precisely, you can fill in the gaps: “Your offer says market price. Is that $10 per pound?”
To handle this tactic in the form of “You can do better than that” simply respond with silence. In America, this is awkward and the argument will often be withdrawn. You may also fill in the gaps, time, place, manner, and price that you desire: in effect, putting words in their mouth when the gaps are there.