We never tried this, and I have doubts it will work, but if you have some time to spent, you can give it a try. Remember, this is not supported by us and you are on your own. You might be slowly corrupting your database without immediately noticing. Having said that, this should do the trick:
First, make sure you always have a current backup of your GarageSale data.
Second, make sure you don’t access the GarageSale library from two Macs at the same time.
You would need to copy the folder Library/Containers/com.iwascoding.garagesale7 to your NAS and replace the original with a symlink pointing to the copy on the NAS.
Let me know how works, but please don’t be angry if it doesn’t.