Mainly not sending, and not just sporadically, but at all.
I just came across an item I really wanted, but the seller has 9 1ratings. They do have a lot of items out there and the reviews are spread across a few months. Then every once in a while do post something, which still brings down the average to something like 2,5.
I get that on ebay sellers get the money and you could almost treat it like a payday loan- the money is refunded eventually, so you at least have it for a bit before it gets refunded to the buyer. But on vinted the money is not released until the buyer confirms receiving it, so why list things if you never intend on posting them?