Make sure there is nothing in your message that could look like a phone number or email address to a simple scan -- that can cause errors as eBay tries to stop such messages from being sent.
Try replying from a different page or link. If you are using a link on one of your selling pages, go to your message inbox and try responding there. Or go to the user's feedback or profile page and use the Contact link to compose a new message.
If you have your eBay messages sent to your email, try replying directly to the email rather than using the blue reply button in the message.
Try going to ebay.ca and replying to the message there.
