That subset is going to be different for most purchasers.
I think this is a problem with the business model, rather than the problem with "unethical" resellers. So you either give up on the business model, as many game devs are doing, or you find ways to enforce it. Either start suing users who resell keys, or partner with the likes of Steam to create a system in which keys can't be activated individually.
If you resell the keys, you're breaking the bundle's terms of service and actively harming the developers involved.
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.
It actually does.
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.
Even something as simple as a checksum of the game key needing to match the last digit of your steam id would solve the problem, and would be trivial for humble/steam to implement. Every time someone requests a key from their bundle, the bundler just had to find a remaining key that matches.
Edit: I originally speculated that perhaps it wasn't in Steam's interest to support this, but thinking about it, they're losing their cut of these dodgy sales.