Is the seller Not A Registered User? That means he has been kicked off eBay, quite likely for repeated offenses like the one you had.
You don't have to worry about feedback. The main purpose of FB is to let other members know what kind of treatment they will get from the member. Since he's gone, it's not necessary.
If he is still a member, you can go to any FB he might have left for you and add a Response. The deadline for FB is 60 days after purchase but Response is allowed indefinitely.