Win some, lose some
Anyway, you've up to 3 months to leave a feedback from the transaction date. Wait till the very last day and leave it. That way, the seller'll not be able to 'reciprocate' your negative feedback.
But then again, a negative feedback affects a seller more than a buyer... So don't be too worried about it on your end.
For my case, I left a negative feedback but he didn't leave any for me...