As already suggested by @fedege96, make sure you have a backup copy of the folder /Library/Containers/com.iwascoding.garagesale/
The easiest way would be to get an external hard drive and enable Apple’s Time Machine for your Mac. If you are the person who forgets to regularly connect the backup drive to your Mac, a more expensive but more convenient option is getting a time machine compatible NAS device (QNAP, Synology, etc.)
Third, there are also paid online backup services, but depending on your upload rate, it might take a while to upload all your changes to the backup companies servers.