Not sure about that, to be honest. But I've never tried trading in-game items, so I don't know if it's safe or not.

Well, it really depends on the platform, but in general, I can tell you that it's necessary to log in to your steam account if you're planning to trade skins. You just need to find a reliable platform for that, and I highly recommend dmarket, just click here and check it out. I use this marketplace all the time, and I'm sure you won't regret it as well.