How can some amazon sellers sell items such as inks and toners on amazon?

I have a few buyers, they are buying Epson/ Brothers / Canons/ Lexmark inks and toners from me and sell on Amazon. I generally sell to them at 55% the price on Amazon. I have been trying to sell them on amazon myself but my requests to sell those items always been denied. From what I have gathered, their accounts are somehow “grandfathered” to sell products such as inks; also, they have some ways to get valid receipts to submit to amazon upon requested.
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