Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. It is about vendor vetting, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
The useful checklist is about verifiability rather than presentation. Independent test results commissioned by buyers rather than by the seller, consistency across multiple batches over time rather than one flattering report, a physical address and a company registration that resolve, batch and lot numbers on the actual labelling, and a shipping practice that matches what the material needs. Everything a good-looking website provides is cheap to fake; none of the items above are.
The condition it depends on
A caveat about community reputation: it is a lagging indicator. Reports arrive weeks after orders, so a supplier can look excellent for a month after quality has already changed.
The practical version
Red flags, in rough order of how much they should worry you: no verifiable address, no batch numbers, prices far under market, vendor-commissioned tests only, pressure toward irreversible payment, and shipping with no temperature control.
What I am not sure about
The question I want answered is how people are distinguishing a supplier having a bad batch from a supplier on the way out. I have searched first, so if this is covered somewhere point me at it and I will read it.
— GraceAZ_72 · corrections welcome and will be edited into this post with credit