Hi, Villager! ;)
That's an awesome question!
To sell a user-created building, when in your village, tap the "Build" hammer button, select "User Buildings", and then tap on "Save a new building".
Select the building you would like to create and sell.
Please note that you can select the ones with yellow linings since those are the ones that you have created yourself. Be careful not to build one item too close to one that was purchased in the store.
After selecting your building and capturing a snapshot of the building, ensure the "Display on shop" checkbox is checked.
Displaying in the shop will allow other players to find it and purchase it in the shop!
We wish you a great time building!
Block Craft Team