The authorisation flow for third party eBay clients is broken at the moment for some (all?) users.
GarageSale uses this flow to download a security token to access your account. Without that token, GarageSale cannot communicate with eBay.
Please refrain from refreshing your eBay access token for now.
Also, new users will not be able to authorise GarageSale at the moment.
Stay tune while we are awaiting feedback from eBay Dev support.