So, I just got scammed...

This kind of problem would be completely gone if steam had an option of redeeming a key as an inventory item - I mean you input the key into steam and get the game as a gift in your steam inventory. Then you could just trade the inventory items and all would be good.

Fill a report, like this.

There is really easy solution for this on steam!!!
1) Steam has to allow to check if the key is valid and not used
2) And if the key is valid there should be a trade like gift trade where you type in the key and the other puts in the gift
3) Both clicks OK and the person gets his key/gift

DON'T TRADE KEYS!!!