Steam

Given that steam let's you sell keys on other platforms (like gog, gmg, etc) and activate them on steam, and have steam handle all the heavy work of file distribution and stuff, it makes sense that steam wouldn't want you to sell steam keys cheaper on other platforms and make them wear all the cost of distribution... Otherwise they'd get no sales and end up with all the expense

The only other choice would be to no longer allow you to get steam keys to sell on other platforms or even to give away for review purposes or things like that.