So first of all I want to refer to the review guideline:
Trade feedback should only be used to describe your personal trading experience with a user. It should not be used (i) to post negative comments when you disagree with someone's offer, (ii) for revenge or retaliation when someone leaves you negative feedback, or (iii) for unrelated comments or trading experiences not directly involving you.
I have a negative experience with this user. Chat log can be seen in the pictures. The trade itself happened and we both got what we wanted.
First of all this trader constantly was saying that he/she is scared of getting scammed because it wasn't a trade that would fully go through a trade offer on Steam. Mine was a HB gift link or key, whatever this trader would prefer although trader first thought it would be a gift (which I didn't state anywhere).
This trader didn't know about HB gift links, so I explained it and sent a link to the HB site where they explain it as well.
Trader again kind of said that he/she was scared of being scammed by asking what would happen if it turned out it would not work. I then offered to send the key directly through the chat. That way I can monitor it better so that I know what key it is and that if trader said it didn't activate correctly I would have the key to compare with the proof of invalid/dupe key this trader would send me then.
All of these questions and suspicions from this trader that I might be scamming him/her made this trader quite suspicious in my opinion.
Trader continues with saying I might be a scammer. Asking me to already get the key (not to send it already though) and then he/she would send me the CS:GO keys. It was already suspicious to me so I told this trader I won't until I am sure I get the keys (as it's harder to get rid of a Steamkey than of a gift link, since most traders think links are safer and such, you know what I mean)
Trader finally sent the keys, but h/she just had to say "RIP 5 keys" implying more and more that I am a scammer.
To summarize, my bad experience with this user has to do with the constant accusations I might be a scammer and the fact that this trader acted like a scammer.