This has pushed me to high levels of brand loyalty with direct ordering from the brands I like and trust, plus a lot of buying used… also mostly products from a handful of trusted brands (this is largely clothes, nearly all of which I buy used except socks, underwear, and most sweaters)

Don't allow 3rd party sellers?

It depends on the brand, but a lot of the stuff I buy is available directly from the manufacturer's site, usually via Shop Pay.