At a legitimate website, the developer(s) and publisher(s) make money for each key purchased and every key is legal to own, so there is little chance of a key being revoked and your account banned on your chosen storefront. At a grey market seller like G2A, they can sometimes get their keys illegally (credit card theft, stolen accounts, etc.). So because GMG, Fanatical, Humble, IndiaGala, etc. are all legitimate resellers, they buy their keys directly from the source (usually the publisher or storefront where they are redeemed, but that’s just my guess). Apologies if that’s not what you were asking.
At a legitimate website, the developer(s) and publisher(s) make money for each key purchased and every key is legal to own, so there is little chance of a key being revoked and your account banned on your chosen storefront. At a grey market seller like G2A, they can sometimes get their keys illegally (credit card theft, stolen accounts, etc.). So because GMG, Fanatical, Humble, IndiaGala, etc. are all legitimate resellers, they buy their keys directly from the source (usually the publisher or storefront where they are redeemed, but that’s just my guess). Apologies if that’s not what you were asking.
I wonder if Humble Bundle has any stance on the giveaways people do with their unused Humble Choice keys
It’s against the TOS unless you are actually giving it away as a gift with no promotional value. https://support.humblebundle.com/hc/en-us/articles/202712380-Can-I-sell-trade-or-use-my-keys-for-promotional-purposes-i-e-Stream-Social-Media-giveaways-
It sounds like the typical Reddit giveaways would be fine under their TOS but they would understandably not provide support for them.