Here is a temporary fix, which ignores if your selected category supports “Best Offer” or not. If you have ever specified Best Offer settings for your listing, this version will upload them to eBay regardless, of what GarageSale thinks to know about its category.
This might cause eBay errors if the “Best Offer” featured is indeed not available in the specified category.
https://downloads.iwascoding.com/downloads/GarageSale_9.4.1rc1.dmg