You have to click on the lock icon in Safari’s address bar next to “api.eBay.com” and set it to “Always trust” as described in our blog. It might be needed to restart Safari as well.
However, it’s strange that you get that error with GarageSale 6.9.7. Maybe it’s a different issue.
Could you post a screenshot of the error and describe when exactly the error appears? (what exactly are you doing?)
Also, you might want to check if you can reproduce the error with the latest beta version of the upcoming GarageSale 7 available here:
Regards, Kristian