There is the FFL/ tax collection end of it too.
The mutual trust thing is a multi layered must. I trust the reloads of my buddies and they trust mine and either direction we know a 150gr .308 loaded with 45gr of Bullseye isn't going to happen.
Some people will pick up obvious (and unknown) hand loads at the range and fire them or buy reloads from estate sales, both of which give me the willys. I have bought a fair amount of pull down powder from vendors, but pre-owned off an individual I would stick to known and trusted too since I can't think of too many brands of powder that are easily confirmed as being unopened.
Primers are less of a concern with the good practice of starting with start loads being as sure as anything on fending off issues.
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