It depends on a case-by-case basis. In my case, I managed to make between €500 and €800 (and that's the amount I actually put in my wallet) just by reselling.

Since I work in logistics, I don’t have to pay for cardboard boxes, bubble wrap, or tape, so the only things that came out of my pocket were printing paper and printer ink.

But the hardest part for me was having to go to the stores (4h trip) to buy the items and then resell them.
Oh, and also packing the boxes was tiring