This is so true. I volunteer in a charity shop, we are only a small unit and have to be picky about what we put out. We don't put out any clothes unless we think we can get £3 or more for them. Fast fashion brands like Shein, ASOS, Pretty Little Thing, Primark, Cider are so poorly made that they are worth nothing second hand. Nobody wants them. Even BNWT they are difficult to shift. We might have more success with heavier weight dresses or winter coats but all the wee summer dresses or tops - just no chance anyone wants them second hand.
Agree with the poster who suggested charity shops for new work dresses - you'd pay £8 - £10 in our shop for second hand M&S, Hobbs, Zara, up to £20 if it's BNWT.