Some items can be sold for Club Money or other items.
Selecting an item to sell will bring up the Sell window.
Next, choose the number to you want sell.
Tap 'Sell' to complete the sale.
However, if selling an item takes you over the maximum Club Money you can hold, any Club Money in excess of that limit will be lost, so please be careful.
*You can't sell a kit or emblem if you have only one left.