If you are still having this problem, can you please give this a try?:
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In GarageSale, choose “Help” menu > “Open Library Folder”
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With the library folder open, quit GarageSale
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In that library folder find the “Synch.leveldb” folder and move it to the trash.
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Start GarageSale and try to enable synching. In the prompt select “Overwrite Data on Server” (if you are enabling synching on your main Mac).