SarahChen_PharmD said:

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

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.

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.

Dr.AddMedPHL said:

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

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

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

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

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

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.