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

Dr.ObesityLA said:

The useful checklist is about verifiability rather than presentation.

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.

hans_munich said:

Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…

This matches mine closely enough to be worth saying so. 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.

From the other side of the consultation, briefly.

Shipping damage protocol for vendor vetting orders: if your package arrives with visible damage, temperature excursion, or broken vials:

• Photograph EVERYTHING before discarding packaging
• Note the temperature of the cold pack (liquid = possibly too warm)
• Contact the vendor within 24 hours with photos
• Request a replacement shipment or refund
• Do NOT use potentially compromised medication

Reputable vendors will replace damaged shipments without argument. If they push back, that's a red flag about the vendor.