Question about European formulas

I see some German brands have become popular in US, but they aren’t fda approved so you can only buy them on some websites.

My question to people who are using them, how do you know the website is legit? There are a lot of scams in food industry, they can for example mix if with another formula, or completely make their own. Wasn’t there that scam in China a few years ago?

I know a lot of people who are doing this but it really freaked me out, since the importer websites are unknown, only other unknown websites or blogs are linking them, even if a reputable site links them doesn’t mean the owners aren’t sketch.