HPLC_Greg said:

The useful checklist is about verifiability rather than presentation.

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.

Happy to go further on any of that.

Adding the numbers, since they settle part of this. 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.

Dr.SportsMedIN said:

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

Adding the part of the answer the thread has not reached. 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.

Following on from Dr.ReproEndo — and this may be the naive question:

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

Closing the loop on my own question.

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.