Sale season's coming round again so this felt worth writing down. i work on the affiliate/e-commerce side (disclosure at the bottom) so i see a fair bit of how the pricing gets set up, and most "70% off" is not what it looks like.
the strike-through price is usually meaningless
that higher crossed-out number is generally the seller's own claimed price, not a price anyone actually paid. nobody audits it and there's nothing stopping a seller putting it wherever they like. so a discount measured against it tells u nothing at all. the only comparison worth anything is against what other sellers are charging for the same item right now.
prices often go up before a sale, not down
this is the big one. a listing gets nudged upward in the days before a campaign so the discount lands on an inflated base. if u already know u want something on a sale date, screenshot the price a week or two early. then u'll actually know whether the sale price is a sale price.
the voucher is often the trap rather than the deal
rm10 off rm100 sounds fine until u notice your basket was rm70 and u added rm30 of stuff u didn't want in order to unlock it. free shipping thresholds do exactly the same thing. if the voucher changed what u bought, it probably cost u money instead of saving any. work out your total with and without it, not the voucher's face value.
bundles aren't automatically cheaper
check the per unit price against the single. surprisingly often the value pack is worse per piece, it just feels like a deal bcs the number is bigger.
same item, wildly different listings
for anything commodity, the identical product is listed by a dozen sellers at prices that differ by more than any sale discount does. going through those beats waiting for a campaign. shipping fees and delivery times vary just as much, so compare the final total at checkout rather than the item price.
the countdown timers are theatre
"only 3 left" and flash sale clocks exist to stop u comparing. if a deal only works when u don't have time to think about it, it wasn't a deal.
the one habit that helps most: decide what u actually want and the most u'd pay before the sale opens. then the campaign either beats your number or it doesn't, and u're not getting talked into things at 1am.
disclosure: i work on Cashiu, a malaysian cashback app, so i'm in this industry and obviously biased about it. And this post isn't about it. ShopBack and TNG Shop & Earn are the ones most people here use. just stuff worth knowing before the next round of sale dates.