Put another way, the structural problem with a group buy is that the organiser typically holds both the money and the material, which means there is no point at which any participant has recourse. That is solvable, and it is solved by design rather than by trust.
Lot-to-lot content variation of nine per cent between two nominally identical lots, both within a stated specification, is the single most common finding in independent testing and the least discussed. It is not fraud; it is the consequence of a fill process controlled to a tolerance rather than to a target. It is also the reason a per-lot content assay is worth more than a per-supplier reputation.
On declarations: the accurate description of a research reference material is a research reference material, and accuracy is both the legal position and the practical one. A declaration that misdescribes the contents converts a customs question into a different kind of question, and it does so on a document with your name on it.
The published aggregate datasets from Janoshik, Medutest and PeptideMeter are the closest thing to a systematic evidence base in this space, and the striking pattern across all three is that identity is almost always confirmed, purity is usually acceptable, and content is where the variance lives.
One qualification: independent testing tells you about the vial you sent. It tells you about the vial you kept only under an assumption of homogeneity that nobody has tested.
Test the first lot from any new supplier, set your accept threshold before the result arrives, and keep the certificate with the lot number and the date in one place.
edited 25 Nov 2024 by g_paskevicius — reworded for clarity after a comment