Hi,
sorry to hear this. As I can not reproduce this, does it still happen if you restart GarageSale? Maybe multiple times?
As I am getting a bunch of crashes here regarding the eBay config that is downloadable from our servers (just verified that it is properly downloadable from here), can you try to delete your last downloaded version from that file and try again?
Please quit GarageSale and delete the file
~/Library/Containers/com.iwascoding.garagesale9/Data/Library/Application Support/GarageSale/sites.data
In your home folder. Afterwards, please start GarageSale again.
The easiest way to remove this file if you do not know how to find it in the Finder is to start the Terminal application and copy/paste the following command. Press press enter afterwards to actually execute the command:
rm "Library/Containers/com.iwascoding.garagesale9/Data/Library/Application Support/GarageSale/sites.data"
Hope that helps,
Paul