I'd imagine Valve is going to have some sort of market fee to prevent shenanigans like this; they'll probably just slap this feature on their regular community market which already has a 5% transaction fee.
Edit: Or better yet just force sell price equal to at-time purchase price when it is bought off the steam store. The law just says they have to allow users to resell games, it doesn't say anything about allowing users to make a business off of it.
Or just let people do act as resellers of your product.
Presumably Valve is doing this hyper-low prices because they have the ability to and it makes the customers happy. They might decide to thumb their noses at everyone and say "okay fine, you guys are the secondary market now, figure it out yourselves, but we have no reason to give games away for free or sell them for 95% off"
Edit: Or better yet just force sell price equal to at-time purchase price when it is bought off the steam store. The law just says they have to allow users to resell games, it doesn't say anything about allowing users to make a business off of it.