You can sell any items you no longer need in exchange for Happy Stars and Rare Metals.
Be aware that whatever you sell will be lost forever and you won't be able get it back again.
What are Rare Metals?
Rare Metals are obtained by selling items with high rarities.
You can exchange them for other items in the Rare Metal Shop.
As a rule, the rarer the items you sell are, the more Rare Metals you will get in return.