I am a charity shop and we get regular donations from two high street brands - things they have tried to sell in their stores and can't. Clothing mostly, but also other things like shoes, accessories, gifts.
Now sometimes the things are great. We had a donation of nightwear recently which sold well, in the main. But often the stuff which hasn't sold in the shops has been reduced, and reduced again, and reduced again, and STILL doesn't sell. Often because of the size/colour/design. We had these absolutely hideous maxi dresses a couple of years ago, size 26, a really ugly sludgy brown colour, weird shiny fabric. The original selling price had been £50, then it had been reduced to £25, £12.50 and the lowest price on it was about £5. Hadn't sold. The market for that product is very niche.
The other brand which donates to us is much more "high end fashion", but puts all sorts of conditions on their donation which means we can't put items in the window, put them on our facebook/instagram page, basically we're not allowed to tell people we have the stock. They are very precious about their brand image.
I would imagine that if stuff hasn't sold in Amazon there will be a good reason for that. Some stuff just doesn't sell however cheap it is. Of course it shouldn't go into landfill, but some stuff won't sell however cheap you price it.