1. Build a cube on land
2. Go to content tab and drag/drop inside content all items what will sale. Items, not boxes!
3. Now go to General tab and set Name and Comment
4. Mark "For Sale:" checkbox
5. Select "Content" with popup menu on right
6. Set price 0$ Zero
7. Check Access right checkboxes down. (All without "Move"!!! Or anyone will move box!)
8. You may add photo from "Texture" tab
9" CLICK "APPLY"

This is all.
Peoples will receive folder named in #3 with content of your box and no need to use sandboxes, restart etc...