Hi,
the App Store version and the website version of GarageSale use different places to store your date. When switching between versions, GarageSale tries to figure out which data set is the most recent and tries to use that, so the switch goes seamless for the user. Seems this wasn’t the case for you.
If you don’t have a (TimeMachine) backup, your best chance is to look for the bigger dataset and hoping your data is still there.
The website version keeps its data here:
/Library/Application Support/GarageSale
The App Store version keeps its data there:
/Library/Containers/com.iwascoding.garagesale/Data/Library/Application Support/GarageSale
There might be also several backup version of this folder, e.g.
/Library/Containers/com.iwascoding.garagesale.bak*
Using the Finder, you need to investigate which of these folders contains the largest GarageSale.sqlite file. Use that GarageSale folder and put it in /Library/Containers/com.iwascoding.garagesale/Data/Library/Application Support/
if you going to stick with the App Store version.
Hope this helps.