Did you try with Firefox in Safe Mode? What security software are you running? Are you on VPN or using a proxy? I've had no issue buying on eBay. see screenshot.

"You don't have permission to access..." errors are characteristic of a firewall that protects the server against misuse and this firewall thinks something is suspicious about the HTTP request. If you use a proxy or VPN then try to use a direct connection. Select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly. *Settings -> General -> Network: Connection -> Settings You can try to clear the Cache and remove the Cookies for websites that cause problems via the padlock button or via "3-bar" Firefox menu button (Options/Preferences) to see if that fixes it. *https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can I've also seen cases where having "Resist Fingerprinting" enabled was causing this issue. *https://support.mozilla.org/en-US/kb/firefox-protection-against-fingerprinting If this all doesn't work then you can try "Forget About This Site" in the right-click context menu of an history item. *https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox