Dr.SportsMedIN said:

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

This answered a question I did not know how to ask. Printing the relevant bit and taking it with me.

Adding the clinical framing, because it changes how the question reads. Whatever the answer turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, and invite the correction. That is slower than asserting, and it is the only version that survives being wrong.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

Dr.RaviCardio said:

Vendor reputation tracking for vendor vetting: I maintain a personal spreadsheet with every order I've placed — date, vendor, batch number, Janoshik…

This is where I part company with the consensus forming above. I think the framing smuggles in the conclusion. Ask it the other way round and the obvious answer reverses, which usually means the question is doing the work.

The figures, for anyone assembling their own picture. One practical note: write down what you did and when, before you need it. Reconstructing a timeline from memory three months later is how people end up unable to answer the one question that would have resolved it.

That is the short version; the long version is somebody else's post.

Moderator note: good thread. Keeping it here rather than moving it, because the question is general enough to be useful. Carry on.