Formula?? OfferUp?!

I have always wondered where do these people get tons of those big cans of formula to sell? I’m not talking about the small 12.4 oz since those are probably from WIC but the other ones? And most people I have bought the big cans from are mostly people without children or even elderly people 🧐🤔 Just my curiosity getting the best of me (& I honestly didn’t know where to put this, sorry) 😅