This is a never ending debate on the eBay forum and really it doesn't matter in the great scheme of things.
Very often sellers won't leave feedback until the buyer has done so, as the buyer's feedback is considered to signal the end of the transaction.
As a seller I always leave feedback first; as a buyer I always leave feedback when the goods have arrived and been examined/tested, whether feedback has been left for me or not.
I have been hassled for feedback before and left - here is the feedback you've nagged me for, as I consider it bad manners to ask for feedback.