There are surprising legal precedents on keys resale:

https://www.gamesindustry.biz/articles/2013-10-09-digital-ri...

It seems that the situation is currently unclear (at least in certain parts of the world), but not obviously illegal.

The problem with resellers like G2A is there's no large legitimate source of keys to resell. They may not be responsible for stealing the keys / breaking terms of service on key sources themselves, but they are acting exactly like a fence for stolen goods, and they know the keys they're selling are almost certainly not legitimately allowed to be sold in this way.

If I bought that key in a bundle, I own it fair and square and I'll do what I want with it. If resellers are helping with that, I view that as a positive.

Except you don't. Every bundle I've been a part of says you can't resell the keys. That's kind of the whole point of the bundle; you're not expected to actively want to purchase every item in the bundle, but the value you get out of it is still worth more than you paid for it, and the developers of the software get to reach users that otherwise wouldn't have bought the game anyway, thus gaining a bit of money from those users and, hopefully, causing increased word-of-mouth sales, or for a multiplayer game causing the player numbers to go up and thus produce better network effects.

If you resell the keys, you're breaking the bundle's terms of service and actively harming the developers involved.

Imagine if they sent you each game on a separate CD and told you "but don't sell the ones you don't want", you'd think that was ridiculous, right?

That's exactly how stupid this idea is. I own the products I purchased, just because it's not a physical copy doesn't mean I have any less rights to it. I highly doubt their ToS would hold up, I consider it highly immoral to attempt to deny people the rights to sell or trade their own purchases.

> just because it's not a physical copy doesn't mean I have any less rights to it

It actually does.