If there’s one thing I’ve learned after years — yes, years — of testing, swatching, scrubbing and slathering on hundreds of beauty products , it’s this: not all sales are created equal. Some are filled with filler (no, not the good kind), while others are an absolute goldmine of cult-favorite formulas, under-the-radar holy grails and splurge-worthy staples at a fraction of their usual price. October Prime Day (Oct 7 and 8 this year) falls squarely into the latter camp. And this year’s beauty