Be sure you are using the actual username, not the store name, which may now appear on some pages or in an item listing where the username used to appear. Go to the seller's full feedback page, then copy and paste the seller username directly from there, being sure you do not copy any extra characters like a leading or trailing space.
Is the seller still currently registered? If not, the report will fail to submit, I believe.
Any difference if you try another browser?
Or if you try reporting the seller on ebay.ca instead of ebay.com? I have had some success reporting item listings on ebay.ca when reporting on ebay.com has failed.
