Instead of just refreshing the existing eBay token, it might be required to renew the eBay token completely:
Remove your eBay account from the GarageSale preferences > Accounts, restart your Mac and then re-add your eBay account and PayPal email afterwards.
If this won’t help, it’s probably an issue with your eBay token saved in the system Keychain. Please give this a try to fix it:
Quit GarageSale if running.
Open the macOS “Keychain Access” application and enter “GarageSale eBay Account” in the search field. (If you don’t find anything try it with “ebay token”.)
Right-click on the eBay account token in the Keychain and select “Delete”.
If done, restart your Mac, start GarageSale and add/refresh your eBay access tokens in the GarageSale preferences > Accounts.