By default, consumers are their own best regulator. You buy only the bread that tastes good, and that shoes that fit, so the market will provide, etc.
It's a small step from there, to outsourcing part of that to a middle men like Costco. Costco benefits from their reputation with customers they got from those efforts.
Similar for product reviews on independent websites or magazines or even on Amazon.
It's only in exceptional circumstances that 'consumer regulators' can not be private entities.
And much of the time, we get lots of government regulation that could be done by private entities just fine or better. But to be honest, lots of that regulation is still 'good enough', so it doesn't do that much harm. Two examples to illustrate:
Germans like to eat raw minced pork. https://en.wikipedia.org/wiki/Mett So there are rules in Germany that all pork sold has to be save to eat raw. In most other countries, people don't share that peculiar preference, so the government regulations on pork are less strict.
I hold that even in the absence of specific regulation, the German market would provide Germans with pork that's safe to eat raw. Companies would just put a little sticker on their meat that tells you, that it's safe to eat (and normal existing rules about truth in advertising would make sure the sticker is trustworthy).
The second example: thanks to harmonised EU rules you can put palm oil in your chocolate without loudly declaring it and still call it chocolate on the packaging. (You just have to mention the palm oil in the fine-print list of ingredients.) By and large, German customers don't like palm oil in their chocolate. So German supermarkets mostly only carry 'proper' chocolate. So we private companies regulating the German chocolate market, to give consumers what they want.