Being impersonated for the first time and I have no idea what to do

Impersonation is one thing - intent to scam with it is another.
SteamRep does not handle anything to do with negative rep or rep issues nor does SteamRep have anything to do with that steamtrades site.
https://forums.steamrep.com/threads/steamrep-investigative-policy.140101/
You can address issues you have there with that site.

This is not the report section, if you want to file a report, go to the report section, read the sticky threads and submit your report.

SteamRep, by policy, does not intervene in other websites bans or statuses, other than banning known scammers as a condition for becoming a friend or partner community. Steamtrades, the site you linked, is not even affiliated with SteamRep (and frankly completely unwilling to work with us), so there is nothing we could do even if we were to try and help clear your name from alleged fake reports.

Steamtrades, as I understand, have taken it upon themselves to try and host their own reputation system based on unvetted positive and negative feedback. Unfortunately, pointing out concerns for abuse, which SteamRep itself has dealt with firsthand, has fallen on deaf ears. They prefer to take a laissez faire approach, where scammers are allowed to roam freely and its users responsibility to review (completely unvetted) ebay-style positive and negative feedback, figuring out who is the liar themselves; while we at SteamRep require airtight evidence, manually reviewed by a trusted and trained community admin in order to brand someone as a (much more definitive) scammer. We take our own approach to trader reputation, and they take theirs; neither of us will ever see eye to eye on that, so coming to us will not make the problem you're having on their website go away. If you choose to rely on Steamtrades' reputation system, which is your right, then getting a soiled reputation once you're impersonated or any time you hurt a scammer's feelings enough for him to brigade your reputation with -rep's (also somewhat common) is just something you'll have to accept as a drawback to the reputation service you use and trust.

If you are concerned about the impersonation, and its impact on your reputation here, rather than your standing with Steamtrades, then I'd recommend keep an eye out for pending reports on your own SteamRep profile, and responding to any that come up. We do expect proof from the reporter that they dealt with the real you, so chances are reports against you will get tossed aside if they don't provide proper evidence. However, having the accused respond to a report claiming it was an imposter is a huge red flag for impersonation, and will tip the admin off to handle the report with extra care. See: Getting "unconfirmed reports off your profile

If you find someone actively and provably claiming to be you - for example, linking back to your Steamtrades profile instead of theirs - then you can fill out a scam report with your own SteamRep block as the "victim", and they'll get marked on our side. Read our special evidence requirements for impersonation with intent to scam first. Bear in mind, Steamtrades prefers a more open and hands-off approach to moderation, and does not ban or discredit anyone for being marked in our database, no matter what they did, so the scammer will still be free to abuse your reputation over on Steamtrades and there's nothing we can do to stop it.

Honestly, we think it's shaky for positive reputation as well. Once Steamtrades reputation is considered valuable enough for someone to go first, the same brigade of scammer alts will throw up fake "+rep" comments, just like shady Greenlight developers, fake Amazon reviewers, and fake Google reviewers. I've heard from credible sources they have mechanisms to catch alts pulling those sorts of shenanigans, but with their refusal to work with or take advice from communities who specialize in this area with years of experience, I'm really skeptical they'll fare any better against abuse than Google, Amazon, and Valve are now. Whether you choose to agree with our assessment or not is up to you, but we think trusting positive or negative reputation on their site is detrimental. They also specifically refuse to list SteamRep status in their "safety report" so as to not discredit any of these reviewers as potentially fake.

If you have proof of intent to scam, per that guide I linked (I encourage you to also review the replies in it), including abusing someone else's reputation, feel free to still report him. You can also make mention he previously impersonated you. It wouldn't be the first Steamtrades impersonator we marked.