Wiktionary
n. A feeling of anxiety or regret after making, or after promise to make, a purchase, especially a purchase encouraged by a high-pressure sales pitch.
Wikipedia
Buyer's remorse (or buyer's regret) is the sense of regret after having made a purchase. It is frequently associated with the purchase of an expensive item such as a car or house. It may stem from fear of making the wrong choice, guilt over extravagance, or a suspicion of having been overly influenced by the seller.
Buyer's remorse is thought to stem from cognitive dissonance, specifically post-decision dissonance, that arises when a person must make a difficult decision, such as a heavily invested purchase between two similarly appealing alternatives. Factors that affect buyer's remorse include resources invested, the involvement of the purchaser, whether the purchase is compatible with the purchaser's goals, and what positive or negative evidence the purchaser encounters post-purchase that confirms or denies the purchase as a good idea.
Usage examples of "buyer's remorse".
As first he'd been overwhelmed with buyer's remorse, feeling embarrassed by the BMW's enormous cost.
Some come back to the store the next morning accompanied by a whack of buyer's remorse.
Let me just say that it's worth my developing a quick case of buyer's remorse and returning the vehicle if I can't get that number right now.