Hello guy,
(dont be rude pls)
I just found this site and I would like to buy some games for CS:GO keys... I already put the steam trade URL here and I have the keys to give them away... but when I click to "Purchase" it's says: "Items in your backpack cannot satisfy the wish list specified." (btw my steam profile and my inventory fully public)
I've heard about I have to put the keys up on this site but where or how does this even works? Please help me!

Most likely CS:GO Item API problem. I remember there is a page (I forgot the URL) which you could check the API/Trade Server status of Steam. If you find the page you could know when you will be able to trade CS:GO keys.