Be careful.
I didn't lose anything but this user sent multiple invalid keys, right after i tell him that i wouldn't redeem the keys for myself.
He claims it was a mistake and wanted to send the correct ones but there was no way I would still proceed with the trade after that.
Trade at your own risk.
Since he was offline for a week I'll assume he is a scammer. If he comes to an agreement to pay my loss I will revert my negative and conclude that he can back up his words.
So, I traded A Hat in Time for his Slay the Spire + Satisfactory (which both turned out as duplicates) and he assured me he would reimburse me. Now, he's gone for a week. He agreed with me to wait 'til I sell those games but I was going out of town so as I sold both games I gave him his game. Unfortunately, later on, they turned out as dupes. It's not a big deal but I am at a 21€ net loss. Luckily the other games he gave (the ones with keys) were not an issue. He also offered gift links.
So, I can't pinpoint the fact whether he is a scammer or not. I would not want to think so however, he might just want to get things much cheaper than he could afford.