'For example my dad's printer broke so he bought a newer model and I sold his incompatible unused ink cartridge in new condition. These things happen but doesn't make it a business.'

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Oh yes, certainly. A handful of un-used bits and pieces is fine (various unwanted Christmas presents spring to mind!)

But ... e.g.   43 x new tubes of suntan lotion or lipstick?   Or 37 x packets of pet flea-treatment?  (you get the general idea...😀)

That's the sort of thing that (in the future)  a useful A.I. system may be able to spot. (I've always been slightly puzzled why ebay can't spot them already.....Seeing as how good they are at spotting private sellers and potential buyers exchanging contacts!)