well, certain supplements use amazon as their distributor which is fine. so in that case, verify that it’s fulfilled by amazon (not a marketplace seller) and then check batch number if you want to be extra sure.

but many folks buy supplements via amazon even though the manufacturer doesn’t recognize them as an official distributor, so you’re just trusting that it’s authentic which is scary imo.

for supplements that aren’t distributed via amazon, in Canada you’ll often end up buying via iherb or well if they’re distributed there. sort of mid-way between a huge distributor and a small shop. again, check batches if you want to be extra sure. 

depending on how obscure your products are, though, they might have all sorts of smaller indie distributors so you end up ordering from random places for certain things.

so, mixed bag.