Lego DC Super-Villains 96% off @ CDKeys

Technically, all of them do "resell" keys. It's just that the official resellers get their keys directly from the publishers and thus the publishers and devs get part of the revenue. And Voidu is an official reseller, that's a fact, not an opinion. ITAD only lists official resellers and Voidu is amongst them and GG.Deals separates official resellers from grey area keyshops and Voidu is amongst the official resellers.

The grey area keyshops on the other hand get their keys from various other sources unknown to the buyer. And these sources range from "bought from official resellers in bulk in bundles or sales" through "acquired by bots in bulk during giveaways" to "stolen by using exploits", "extorted from devs" and "bought with stolen credit cards".

And it gets even more obscure on marketplace platforms like Kinguin, Eneba, G2A and so on, where you don't even buy the key from that keyshop but from a third party. With these grey area keyshops, you can never know for sure if you just partook in a crime or if you simply bought a key that someone else bought legally from official sources.

Of course the same goes for SteamGifts. You can never be absolutely sure about how the key you won was acquired. It might be highly likely that a key for a game that currently is in Humble Choice comes from there but there is no guarantee.