My mother always said "50% off what?" Obviously when the merchant sets the nominal "retail" price to whatever they want, they can set the discount to whatever they want too and maintain their profit margin. This is basic algebra solving for one variable given a desired "discount". It's amazing how effective it is though.
I'm only (vaguely) familiar with the rules in the UK, but they include tests to ensure the 'was' price was actually a genuine usual selling price. This includes tests that the 'full' price must have been in place for longer than the promotional period, that it must have been recent and that significant sales were made at that price. Buy one get one free also has to mean you get two equivalent or similar products. Compliance is also monitored and enforced. I'm not claiming it's perfect, but by and large over here you can have reasonable confidence these promotions are usually valid.
Conversely my wife is Chinese. Over there 'buy one get one free' usually means something like buy one dress, get one flimsy, non colour-matching belt free.
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