If you're doing this sort of thing, usually you batch your orders together (ideally with some friends) and randomly lab test a few vials selected from the large batch you receive. The powder form vials you receive before mixing will last more or less indefinitely in a freezer, so you can order multiple years worth of supply at a time.

The potency and purity are typically pretty good but from time to time vendors will be outed for either outright fraud or a bad batch that is underfilled or low purity. Most of the direct vendors (out of China) will guarantee fill and purity, and also give credit on your next order for lab test costs if you share the tests with them for future marketing purposes.

All sorts of risks involved of course, but you can mitigate them fairly effectively. Your risk tolerance may vary. Some folks will buy single vials from US resellers and trust those seller's lab test reports. Others (smartly, imo) don't trust that and do their own testing directly on the product they receive.

This grey market supply chain is remarkably robust these days. Plenty of suburban Facebook soccer moms organizing group buys.