In order to sell items to CeX online, you need to be 18 years of age or older.
You'll need to add an official government ID, with a total of 5 points*, to your account prior to creating a Sell to CeX order.
You can update this by going to Account > Your Identity Details
Alternatively, you can provide your passport alongside a proof of address (a rental agreement, internet or phone bill, bank statement etc.)
*5 points of ID must include: Name, Address, Date of birth, Photo and a Signature. Examples: Drivers licence, state proof of age/photo card