Someone recently sent me an offer, which I ended up accepting. After I did and gave him my address, he said that he was still negotiating. I thought this was strange since it makes no sense to send an offer when you want to negotiate. He was able to cancel somehow, which I thought was problematic. Do I have any action, or should I just avoid this person?

What would you want to do? I think avoiding is the best course of action. It's a bummer that you released your address and the trader cancelled. I would be interested in what the reason for cancel was.

His reason was "I was under the impression we were still discussing the premium", since he wanted a premium for a card that he described as having a mark on it. I think he felt like he wanted to call it a misprint and get value from me because I like them.