LabKate 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…

That is correct as far as it goes, and here is where it stops going. Prices dramatically below market are the strongest single signal, and the reason is arithmetic rather than suspicion. Synthesis, testing, and cold-chain shipping have floors. A price well under the floor means something was skipped, and the two things that get skipped are testing and content.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

LabKate 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.

This one has a reasonably settled answer, so here it is. 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.

labquiet_amy said:

Prices dramatically below market are the strongest single signal, and the reason is arithmetic rather than suspicion.

Adding a me-too, because a thread of one person's experience is not much use. Posting only so the count is not one.