Don't stock up because of "crossed-out prices" and big sales

Moderate value
Evidence C
Saves money
Free
Takes minutes
Some willpower

In plain words

When people judge whether something is expensive, they rely on the first number they see, and later adjustments are never enough. So "original price 999, now 499" makes you systematically overestimate how cheap it is. And by regulation, the base that a discount is measured against is only the lowest transaction price at that store in the 7 days before the promotion. The crossed-out price may never have actually been charged.

Cost: Free. The price is that you also pass up some real discounts. You have to remember the everyday prices of the few things you often buy. The hard part is holding back from stocking up on the day of a big sale.
What you get: People's judgment gets anchored by the first number they see, and later adjustments are usually not enough. So "original price 999, now 499" systematically raises your estimate of "how much cheaper it is." The regulation says the base for a discount should be the lowest transaction price at the same store in the 7 days before the promotion, so a "percent off" is to be measured against the lowest price the store actually sold at in the 7 days before the promotion. The crossed-out price may never have actually been charged (China, in effect since 2020)
Notes: The anchoring effect is a conclusion from lab studies, and no one has calculated how much money stocking up actually loses. Stocking up also has costs you won't write down: things expire, take up space, and "since there's plenty at home, use a bit more." Stock up only on things that meet all 3 conditions: a long shelf life, certain to be used up, and a unit price truly lower than before.
Evidence grade: C : The author's experience or common practice, without direct literature.
Sources: Tversky A, Kahneman D (1974). Judgment under Uncertainty: Heuristics and Biases. Science 185(4157):1124-1131. https://doi.org/10.1126/science.185.4157.1124
国家市场监督管理总局 (2020). 规范促销行为暂行规定（总局令第 32 号，第二十一条）. https://www.samr.gov.cn/zw/zfxxgk/fdzdgknr/fgs/art/2023/art_cae53a080be2401e8f91c6d6291539f8.html

Translated from the Chinese original. Original book, section 5, tip 24. This section is about money and spending. It is general information, not financial or investment advice. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer