LindaRN_retired said:

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

LindaRN_retired has the substance of this right. The condition it depends on is worth stating. 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.

LindaRN_retired said:

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

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.

Short answer first, then the reasoning. 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.

Ask again with the specifics and you will get a better answer than this one.

KevinCompounds said:

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

Agreed, and mass spec answers a question purity does not: identity. High purity tells you one thing dominates the chromatogram, not that it is the molecule you ordered. A mass within about a dalton of expected is the identity check.

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