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.

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

Why a shortage listing created a legal pathway at all, since a shortage is a supply fact rather than a permission?

Dr.PulmRoch said:

Red flags, in rough order of how much they should worry you: no verifiable address, no batch numbers, prices far under market, vendor-commissioned…

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.

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

Correct me if the detail matters more than I have assumed.

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

The bulks-list asymmetry was the piece I had missed entirely. It explains why one of my two pharmacies is still arguing it can supply and the other simply stopped.

pat_auckland said:

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

pat_auckland has the substance of this right. The condition it depends on is worth stating. Worth adding the genuine exception, because it is real and narrow: a change made for an identified patient where the prescriber determines it produces a significant clinical difference for that patient. A grid of fixed doses offered to everybody is not that, whatever the intake form says.