Are you using the latest version of the eBay app for your OS?
If the app is not allowing you to view your messages, see if you can use a browser instead.
Mobile browser; mobile browser in desktop mode; or a desktop browser.
If your messages inbox is not loading, try another browser or an alternative link, such as this for Member to Member messages:
https://mesg.ebay.com/mesgweb/ViewMessages/0/m2m
Or try signing in to ebay.ca and reading your messages there.
