But then the seller is missing out on those $50, which they can't spend anywhere else. From an macroeconomic perspective, the effect is zero. Like advertising, it only serves to allocate resources, not to create value. What's more, it's mainly large companies that benefit from something like this. Firstly, because of scale effects, and secondly, because they can sustain price dumping for longer than smaller companies.

The same applies to deals. You only benefit from them because someone else is disadvantaged. Unless, of course, you assume that companies have something to give away out of kindness. And, of course, you yourself have been someone who has given someone else an advantage at your own disadvantage. You paid more so that someone else could pay less. The macroeconomic effect was zero because no value was created.