It seems this user registered on Steamtrades just a few hours before I read their offer (I think they pretty much registered and sent it). Long story short, they added a comment on my thread (asking for a trade) and then left a -rep, claiming that I had scammed them. Unfortunately, I hadn't even replied to their offer yet, nor had I added them to my Steam friends list (and vice versa), so I suspect they've been scammed by an impersonator. After noticing the -rep and finally reading their comment, I replied here on Steamtrades and also left a comment on their Steam profile, in order to know which Steam profile they dealt with (and possibly find out the impersonator). I'm open to providing help to solve this issue, but I haven't received any response so far (neither here nor on Steam), so at this point I don't know if they want to solve this together, or they're just a troll who wanted an excuse to leave a -rep on my profile.