This account appears to have been used for fraudulent activity

Hey!
Two days ago, I've got this notification.

This account appears to have been used for fraudulent activity.
This account will have restricted functionality until this issue is resolved.

Right now? I'm tradebanned. Not sure if its just temporary or permanent.
I also have a good idea for what I got this ban.
A guy added me via steam / I guess he found me on golounge / reddit, and told me he wants to buy my awp asiimov WW stattrak. He had no keys, so he asked me to put a 2c skin on the market for 160ish euros. Thats like 61 keys / wayy too much. I was pretty sure he was a scammer, and would ask me any second to go first. I also asked him, why he wouldn't just buy it on the market, and he told me, that he needed the item to be tradeable. So I thought, if he doesn't want to scam me, he hijacked this account, and just wanted to transfer the money away. All of the sudden, he bought my 2c famas, and I had 140.60€ on my account.

Here comes my poor desicion: Since I was sure he was a hijacker, I was like: Thanks for free GTAV for me and a friend of mine, deleted and blocked him and gave him nothing.

So lets sum up what happened:
--he added me
--he acted suspicious
--he did all the talking, I never agreed to anything
--he bought my 2c skin for 160€ (-13% for volvo)
--I deleted him (so I guess if it was really him, I scammed him)

Here are my questions:

Anyone experienced a case alike mine?
Is the steamchat equivalent to a contract?
Did I violate the trading TOS?
Y I CANNUT TRADE MY SKINZ?

Thanks for your time, and thanks for reading the post!

PS: What I also forgot to mention: I got a stash worth 3000-3500€ on this acc ((((((((((((((((
PS2: Sorry for bad english, I'm austrian