Are you replying using a link in an email? Try going through your eBay message inbox page instead.
https://mesg.ebay.com/mesgweb/ViewMessages/0
Are you able to reply if you go to ebay.ca or ebay.co.uk instead?
One thing that usually works for me is to go directly to a user's feedback or profile page and using the "Contact" link there to compose a new message.
