Can this person take me to the disputes tribunal?

I sold an iphone 4 that i bought off someone else, it has a problem and i listed all i knew, in the auction it was clearly stated SOLD AS IS, now the person after a week of recieving the item he wants a refund, and is thretening to take me to the disputes tribunal. What can i say or do

As long as you stated "as is" there isn't anything they can really do. The "as is" is pretty much a buyers risk