I went back and looked at his feedback. The first site he won, the seller sent negative feedback, telling other sellers not to sell to this guy. The next two sellers gave him positive feedback. But I did notice that he hadn't paid for his flippa fees.
Once the full 3 days of waiting ended, I filed a dispute. Apparently I had to wait another 3 days to give the winner a chance to reply. He never did. So Flippa relisted my auction and waived fees.
So what should I do about the feedback? Should I send a negative feedback to him and risk getting one in return? or should I leave it be and move on?