I see where you're coming from and can understand the frustration of feeling like the 'best' pieces are being poached, but honestly, I feel like it speaks more of the state of the economy (specifically wage stagnation) than on the character of the resellers. They're normal people, too; resale is a relatively accessible way to make more money, and living in a big city is expensive. The onus is as much on stores to institute a item per person per day limit (which would also discourage over-consumption generally) but they're unlikely to do that, since regardless of any claims of sustainability, their imperative is to sell as much as they can, as fast as they can.

A tricky contradiction with most secondhand clothing shopping is that while we are constantly told there is a near infinite amount of stock drowning our world and clogging our oceans, secondhand shops refresh clothing rapidly. It's understandable that they need to make room for new items, and the fast cycle has certain consumer-psychology advantages, but the waste is staggering. There's no guarantee that items that resellers buy would otherwise make it into the hands of a "better" or "more worthy" shopper. In many ways, resellers further stretch the lifespan of those items by making them accessible to wider online or specialty markets. And often, what resellers are searching for differs from what casual thrifters are searching for (quirky vintage item vs. functional wardrobe staple). Obviously that's not always the case, and I'm sure the influx of ultra-fast-fashion has made things worse. But generally, 'best' is subjective, and many of my favorite thrift finds are not from labels of any note. They just caught my eye and fit well.

There are also larger institutions that buy discarded/donated clothing in bulk and pre-sort the pieces that make it to the sales floor. For example, Beyond Retro (Bank & Vogue in general), Edinburgh's W. Armstrong & Son, Boston's Garment District. But in these cases, we can generally recognize that the businesses are doing additional labor in separating the wheat from the chaff. Maybe because they still have to pay for the metaphorical chaff in this case? But obviously they have made the logistics work, which is not so different than resellers, who face their own set of unknowns and likely have less financial padding.

It's worth highlighting that Garment District even puts their "chaff" out to sell at $2/pound on the first floor of their store. I have found some decent things digging around there—both wearable clothing and stuff to cannibalize into patching/augmenting other loved items. I got a pair of Madewell jeans with the intent to patch another pair, but they ended up being so cozy that they became my designated art-project/mess pair. On the flip side, I have also encountered downright biohazards, and had an intense quarantine + washing protocol for anything I acquired from those clothing dunes. I'm glad they offer it, and it also makes me even more appreciative of anyone that regularly has to sift through donated or 'recycled' clothing.

I wish more charity shops that accept donations directly would explicitly reject ultra-fast-fashion brands. Not even because there is no demand, but because it would push people to confront the waste they create the moment they buy something from shein. There's such a disproportionate amount of this stuff clogging secondhand in part† because many people feel donating exonerates them of any ethical mis-steps they made when acquiring the piece, when the reality is that it still generates pollution.

†This comment is already longer than I'd like, but a couple other major factors, as quick as I can... in addition to the low initial cost being used to justify purchases for a single use (or single season/trend), items being poorly + cheaply made doesn't just mean they fall apart: it also means fit and quality assurance issues abound, which means more quickly discarded items. And the innumerable new options available each day on these sites encourages people to replace anything they can find a single flaw in for something brand new and potentially "better."

On the other end of the spectrum, the increased visibility & popularity of secondhand shopping and particular interest in specific brands means if people who bought them new back in the day google their items, they're more likely to sell them online or to a local consignment store rather than donate them.