Hi garjus,
You need to unequip the equipment you want to sell first. You can do that by going to your profile and clicking the EQUIPMENT button. From there, click EDIT EQUIPMENT and it will load the game client and show you your inventory.
Once you are there, unequip the clubs you wish to sell by dragging them down into your inventory. You can then drag your starter clubs out of your inventory to fill up the empty spots. Be sure to click the SAVE CHANGES button when you are done.
After that, go back to your profile, click the EQUIPMENT button once again and then click SELL EQUIPMENT. You'll be able to select and sell back the clubs that you just took out of your bag.