The campfire is a decorative item.
To purchase it go to Tools > Shop > Decorative. Then scroll down to find the campfire.
I believe there is one campfire on the map so you have to purchase 9 more at 200 coin a piece. They don't serve any real function so you can sell them back and free up space once the achievement unlocks.
To sell items to go Tools > Move then select the item you want to sell.