Answering the narrow version, because the broad one does not have a single answer. 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.

mike.trainer_LA said:

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

Agreed, with a caveat about community reputation: it is a lagging indicator. Reports arrive weeks after orders, so a supplier can look excellent for a month after quality has already changed.

BariatricNurseD said:

Living somewhere the domestic route does not exist, which makes most of the advice on this board inapplicable to me.

Right, and one flattering data point from a group buy is not consistency. Consistency means separate batches, separately commissioned, over months.

Clinical perspective, offered as context rather than as advice.

International shipping tracking for cross-border ordering orders: my experience with customs in the US when ordering from overseas:

Seized and destroyed — had to get a reshipped package.

Tips: always include a copy of your prescription, declare medication properly on customs forms, keep quantities reasonable (personal use = 3 month supply max), and use a vendor who has experience shipping to your country.