The scarcity heuristic is a mental shortcut: things that are hard to get are assumed to be more valuable, because throughout most of human history, scarcity genuinely did correlate with value (food, safe shelter, mates). Marketers and salespeople exploit this correlation directly by manufacturing artificial scarcity — 'only 3 left,' 'offer ends tonight,' 'exclusive access' — even when the actual supply is unlimited, because the psychological reaction fires the same way regardless of whether the scarcity is real.
Scarcity's pull is amplified by loss aversion: a limited-time offer isn't framed as 'you could gain this' but as 'you could lose the chance to have this,' and losses are felt roughly twice as intensely as equivalent gains. The practical defense is separating two questions that scarcity tactics deliberately blur: 'is this actually scarce' (often not, since the same 'limited' deal reappears next week) and 'do I actually want this at this price independent of the countdown timer' — answering the second question first, before ever checking the timer, short-circuits the tactic almost completely.
Kahneman recounts that he and Tversky's foundational work on risk began with a deceptively simple question: offered a certain loss of $100, or a coin flip between losing $200 and losing nothing, most people take the gamble to avoid the certain loss — even though both options have identical expected value. The reason, they found, is that losses are felt roughly twice as intensely as equivalent gains. That asymmetry, formalized as prospect theory, was cited by the Nobel committee when Kahneman won the 2002 Prize in Economic Sciences (Tversky, who died in 1996, was ineligible since the prize isn't awarded posthumously).