Last time I checked I thought that was eBay's policy. They encourage you to leave feedback but it is not mandatory. To not leave feedback, therefore, is acceptable to eBay, and someone who chooses not to leave feedback has done nothing wrong.

I buy/sell on eBay. The Feedback system is overrated. It can only expose the very worst offenders on both sides, after that, it is of little use.

I cared for the first twenty or so transactions. I still update feedback, but it feels like a chore now.