TrialNerd_Beth said:

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

Filing a mild objection. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit. Import rules are jurisdiction-specific and this board keeps giving US-shaped answers to non-US questions. What is a personal-import allowance in one country is a controlled-import offence in another.

The figures, for anyone assembling their own picture. 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.

Dr.BariatricHTX said:

Import rules are jurisdiction-specific and this board keeps giving US-shaped answers to non-US questions.

Coming at Dr.BariatricHTX’s question from a different direction. 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.

A narrower follow-up, since the general answer is now clear:

What a vetting checklist should actually contain, as opposed to a list of things that are easy to fake?

OP back with an update, since a thread like this is useless without one.

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.