A website cannot inject your computer with a keylogger (not without me actually downloading and executing a file)... It can however trick me into logging in again thinking I was on the real website. I have ran scans, malware, etc... My computer is squeaky clean and I have never in my life suffered a security issue like THIS before. That is why I am pretty sure, the only way(s):
1) I got phish linked.
It wasn't that long ago. Can you try and remember if the seller sent you a link and asked you to check out a thread for whatever reason? Why would there be a need for you to click on such a link? Try to remember the conversations you had with that person that got you tricked.
A phishing link requires you to submit your login details on the fake site. If you spoke with the seller or did some other forum activities one minute and then you were asked to log in again the next minute after getting a link from the seller, that should be something you would remember. Your computer being clean and malware-free is normal. Phishing links are not flagged as malware.
But what about your browsing history? Go through it thoroughly. If you got a link from the seller in PM, the fake site that disguises itself as Bitcointalk has got to be there. Check your browsing history for the day or days (if there were several of them) when you first started talking to the seller over PM and see if there is anything there out of the ordinary.
