Who counterfeits breakfast cereal??? I've had this issue with both Quaker Oats and Raisin Bran. The scale of fraud is magnificent. And it really gaslights consumers; it took me forever to actually accept that I was eating an off-brand imitation of my favorite cereals.
I do still buy from Amazon. The ease of returns still gives me confidence that I can decide after receiving an item whether the price:quality ratio is worth it.
But not if I absolutely need it to conform to certain standards and I don't have the skills/tools to validate the performance myself (like HEPA filters). Then I tend to buy from somewhere with a more curated supply chain (Home Depot, direct from manufacturers website, etc).