To sell your clubs from the mobile app, press the three white lines on the top left of the Main Menu and then select ProShop and then select either BRANDS or CLUBS from the header. Then select "Sell Clubs" on the far right. You will then be given a list of your equipment and the option to sell most non-equipped items.
To sell clubs on PC: click on the EQUIPMENT tab from your Profile page on the website. From there you will see a button titled SELL EQUIPMENT. Click that button to begin the process of selecting which items to sell.