So I didn’t get scammed because I had a bad feeling. Anyways someone pmed be an offer off 2250 for soemthing, they only had 2280 so first of all I was a bit worried. Anyways we’re in game and a tell them to post on my topic so I could request the mediator. He left the game and hasn’t talked to me since.

I’m aware they did nothing wrong, but I’m worried this person may scam someone in the future. Can I post his username somewhere or just leave it alone?

You did the right thing by asking him to post and to request a med, but just because he left doesn't mean he was trying to scam. Maybe he doesn't like trades to take long and didn't want to wait for a med, same with not wanting to explain that so he just left to find another trade to save time.

Not worth reporting as all he did was leave.