I see you are registered in Australia. Are you trying to send messages on ebay.com or ebay.com.au? You should be able to view your messages on either site, but there may be some selling functions that do not work on other country sites than the site you used to list your items.
Do the messages appear in your messages inbox? Can you reply from there, if you were previously trying to reply on another page?
Can you compose a new message to that buyer, rather than a reply?
Are these buyers still registered? If you received an offer or a message from an account that has since been removed by eBay, you might receive an error when trying to send a message to that account.
