That's been my experience so far when getting electronics from costco. Seems like they do a decent job of curating decent stuff.
So vendors won't ship them shit, because they know they'll eat the returns.
I'm sure they do additional curation on top of that, but making them stand behind the product probably gets 80% of the way there.
So if random brand X wants to keep selling those toilets at Costco, they will sometimes get a deduction from their invoice because a curmudgeon brought in a ten year old toilet that broke.
(Costco had to change this policy on computers and TVs because word got out and people began abusing the policy so much the vendors revolted.)