Large marketplaces don't have a single "price": they have an algorithm. What you see right now can differ from what was shown an hour ago, or from what another shopper sees at the same instant, depending on demand, competition, time of day, stock levels, and even the device you're browsing from.
Dynamic pricing exists to maximise margins, not to do buyers a favour. A price can climb just as a product is running low on stock, or drop the moment a competitor cuts theirs in real time.
The most reliable way to avoid getting caught out is to look at a product's price history rather than the price on screen right now: an item marked 20% off can still cost more than its average price over the last three months.
That's why we compare the same product across multiple retailers instead of watching just one: a "flash" deal at one store often coincides with a lower price that's been stable for weeks somewhere else.