This bug was fixed in Patch 1.5.149
My Sim worked on his retail lot all day and sold a lot of things, then went home for the night. The next day the lot was reset to how it had been before the work day and all progress is lost.
Trying to save sometimes leads to save error 531.
The exact cause is unknown but evidence suggests that this is related to removing signs of sold wares, especially if you decorate your baking goods.
I fear that save games saved with this issue can’t be saved. It might be possible to bulldoze the lot and rebuild or to load an older save game.
How to Prevent
I would recommend to not remove the signs and not decorate your baking good for the moment.
You can either remove the sign through build mode (not possible for baking goods though) or restock your wares instead of putting new ones (and yes, I’m aware that you will have less profit because of this)
Wares that were sold have a light orange color in build mode