LondonLisa said:

Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…

No disagreement with LondonLisa. One condition attached. The pattern that distinguishes a bad batch from an exit is behaviour rather than product. A bad batch comes with communication, a reshipment offer and a batch number. An exit comes with slower replies, pressure toward less reversible payment methods, sudden discounting, and the same reassurance repeated without any new information. The product tells you less than the correspondence does.

LondonLisa said:

Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…

I disagree that testing history is decisive. It tells you what a supplier did when they were being watched. Continuity of behaviour under stress — a late shipment, a failed test, a complaint — is more predictive than any run of good results.

Answering the narrow version, because the broad one does not have a single answer. 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.

COA_Karl said:

The pattern that distinguishes a bad batch from an exit is behaviour rather than product.

Second this. I had assumed I was the exception until I read this.