Please open the GarageSale preferences > Accounts and refresh the eBay access token for your default eBay account. A valid token is also required by GarageSale to download the latest category data from eBay.
If this won’t help, 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.