Website for steam key trading

Hey guys

I'm not sure if that's the right place to ask, but as this is a game-dev forum, maybe someone has some info on this:

I had an idea about making a website, where people can create accounts, add games for which they have leftover keys from hubmlebundle, or some other source, create a wishlist, and the website would automatically suggest them trades with other users based on their games.

Would that practice be legal or could i get in trouble with steam, humblebundle etc? (Besides the possibilty of scammers)

I wouldn't want to have a payment system. Either just contact data, that users can specify or a form where users can exchange their keys.