A reference post rather than a discussion. Corrections are the point; I would rather this be right than mine. 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
What I am trying to establish is how people are distinguishing a supplier having a bad batch from a supplier on the way out. Tell me what I have not thought of.
— amsterdam_pete · corrections welcome and will be edited into this post with credit