Dr.GastroMayo said:

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

Pushing back on Dr.GastroMayo here. The affordability discussion here usually stops at individual tactics. At list price this class is out of reach for most of the people who would benefit, and no amount of appeal strategy changes that — it is a pricing problem wearing a paperwork costume.

One concrete data point for the thread. 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.

Worth separating that from vendor vetting, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.

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

Dr.PeteFamMed said:

The affordability discussion here usually stops at individual tactics.

Coming at Dr.PeteFamMed’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.

One thing that is still open after maya_sedona’s answer:

How people are distinguishing a supplier having a bad batch from a supplier on the way out?

Closing the loop on my own question.

Follow-up — getting the denial reason in writing was the step that mattered. It named the criterion, and the criterion was a document I already had.