There are multiple payment options with CEX.io. The two main ones are Credit or Debit card and Bank Transfer.
Credit and Debit card purchases are charged a 3.5% fee. Bank transfers have no fee.
There are different verification levels for card and bank transfers. The two main pieces of information will be your ID and your proof of residency. They will ask for a selfie with your card if you choose that option.
Working to get more information before providing a full CEX.io review.