Is there a way to verify if a vendor has been FDA warning-lettered?

Short answer first, then the reasoning. 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.

BenResearch_OR said:

The pattern that distinguishes a bad batch from an exit is behaviour rather than product.

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

DadBodDave said:

Eight months with one supplier, then a change in how they communicate that I noticed before I noticed anything about the product.

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

Clinical perspective, offered as context rather than as advice.

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.