please kindly help me report this scammer

ah ok, i´m sorry^^ i was wondering if there would be a big gap between both times but yeah, this speaks not hin his favour.

1) That should be a red flag for you

2) Oh, okay, didn't know that (Never used it myself, I'm just aware that it exists).

3) It's pretty hard to "guess" a steam key, because they are not made of only digits, they are a mixture of letters and number.
There can be 221,073,919,720,733,000,000,000 of them, so it's almost impossible to get a right one.

Also, just a thought: If a person can ask the support, when was a key activated... wouldn't they, if provided an unused key, tell you that it was not yet used?

There are 221,073,919,720,733,357,899,776 combinations of 15 symbols (each one is one of 36 possibilities). And I'm sure there are other types of Steam keys apart from the 15 symbols one. So you can be sure no matter how old your key is, nobody has guessed it. And no, I'm not kidding.

Edit: I'm pretty sure that's right, but since it's different from the other reply, the number doesn't matter, the point is nobody's randomly guessing your key.

I think it's not good to call out anyone.
I did that in past :<
But I cannot find guideline that specifies this. Maybe I try wrong keywords.

When you post something which violates site rules, it does not matter in which sub-forum you post it.

The latest version of SteamGifts' Guidelines specifically states...

• Private or identifying information. Users in the community have a right to privacy. Do not post their private or identifying information without their consent. For example, their name, address, phone number, photos, or private messages.

If you obtained this person's consent to post his or her private messages on our public forum, your post is fine. If not, you are violating our site's rules. What you are doing is referred to as "calling out," something which used to be explicitly forbidden on this site and which is still discouraged.

If you wanted help "naming and shaming" a particular person, please do not do it on our website. If you want to warn other people about a scammer, do so in a "general" way rather than including personal, identifying information. For now, you should remove the identifying information contained within your original post. I am giving you an official warning for this particular violation. Future violations may result in corrective action by SG Support Staff. I suggest you review our site's Guidelines and FAQ.

Getting back to the topic at hand, this "trade" is full of odd things.

1. Normally, money goes first. If the key is part of a purchase from an established source (e.g. a well-known key trader), then money always goes first, period.
2. If the key is "bad," it is the key supplier's responsibility to replace the key. Usually, that includes revoking the old key.
3. Swapping or borrowing someone else's Steam account is almost always a bad idea, and is most often connected to "dubious practices." Not only does it invite theft and sabotage, it also makes it more difficult to resolve any problems that arise.

While +4 may be positive, it's nowhere near enough to be considered as trustworthy. The fact you're using a "friend's" account out of fear of bad rep just says that you shouldn't be trusted.

Your main error though, from what I can see, was saying you got the key in a trade and that it may be a dupe. That just screams "scam me". Your uncertainty is another person's opportunity.

yeah but sometimes this can result on accounts suspended for no reason because why would someone go on the internet to tell lies, right? besides, this isn't a trading site so we shouldn't really care about this to begin with

I don't think you understand their point. Imagine I make a post falsely accusing you of something and asking people to report you, even though you didn't do anything wrong, would that be fair? That's the reason the rule exists, you can't just take someone's word like that.

Not like we can do anything here either way. This is a gifting site, not a trading one.