How do I make a shop that buys skins for the player?

How do I make a shop that buys skins and saves for the player? And they can be selected and used anytime?

In the general sense, you use the IAP object : https://www.scirra.com/manual/173/iap

— I mean with in-game money

You would use a global variable for cash I guess. When you want to buy an item it subtracts the amount from the global variable. In terms of unlocks you could look into using arrays to set whether items are owned.

global variables + local storage