TL;DR This person is a complete scammer. Do not trade with him as he will try to scam you every step of the way.
At first he approached me asking to buy 3 payday 2 DLCs I had. I asked for 15 cards for all 3, to which he agreed.
To avoid misunderstandings I try to make sure we're both on the same page about card values. He plays dumb and haggles down the price to 12 cards. I agree and verify with him he understands what value cards we're talking about - min value of 0.08$ for each card. I send him the humble gift links to the DLCs, he proceeds to send me 12 cards valued between 0.03$ and 0.07$ (mostly around 0.04$-0.05$).
I call him out on this, and tell him he needs to send me cards with correct values. In response, he sends me 2 XX% off promotions and a card pack.
I call him out on it again, and suddenly he decides he doesn't want to trade anymore and tells me he didn't redeem the gifts links I sent him. I check the gifts were indeed not redeemed (at that moment) and cancel his trades (I never accepted them).
At this point I still give him the benefit of the doubt. After all he does have a spotless record on ST with 25 +reps (in retrospect, most if not all of his +reps seem to be based on idling games for people, so it's unsurprising he got so many +reps). Maybe he's just that stupid, or doesn't understand English.
The next day, I have some free time, I decide to make sure he didn't redeem the gift links while I slept, and redeem them myself just in case. Lo and behold, all 3 DLCs are suddenly redeemed!
I contact him again give him a chance to make it right before I report him. I understand now his behaviour was malicious all along, but I'm trying to minimize my losses by trying to convince him to do the right thing. At first he (unsurprisingly) claims it's not him and he didn't do it. Next he admits he "suddenly" has the same DLCs he stole from me, but claims he bought them from someone else. And eventually he tries to placate me by underpaying (yet again) for my DLCs.
Well, I actually prefer to lose my DLCs to this thief, but warn others about him.
In summary, how do I know 100% he stole them from me:
I did not take it because I said,
take back your link if it is too heavy/hard
and indeed I have no intention to take the DLC.
and I unfrieds him. (Trade had been canceled)
nah ... on my computer chat history has been erased.
he added me back. and pitted, the DLC has taken me.
I am also confused, how I was accused by him.