It is very simple to buy and sell items using ScriptInterface#Shops.
List<Item> ShopItems - Gets a list of items in the last (or currently) loaded shop.
bool IsShopLoaded - Checks whether a shop is currently loaded.
Load(int id) - Loads the shop with the specified id.
BuyItem(int shopId, string name) - Buys the item with the specified name from the shop with id shopId. This will load the shop first.
BuyItem(string name) - Buys the item with the specified name from the last (or currently) loaded shop.
SellItem(string name) - Sells the item with the specified name from the player’s inventory.
LoadHairShop(int id) - Loads the hair shop with the specified id.
LoadArmourCustomizer() - Loads the armour customizer.
These methods are very easy to use.