Just started today Sunday November 15, 2015 “My seller ID is not valid” is eBay-automated response when I try to reply for feedback left for a purchased Item!
Unfortunately, you are not the first person to report that particular error when attempting to do that particular thing. I think it's been about a week now, maybe more. It's a technical error although I don't know if it's one that is sporadic or ongoing. You'd have to call Customer Service and ask. Let us know what they tell you if you decide to do that. Good luck!
