Data compromised by the hackers includes first names, surnames, addresses, email addresses and phone numbers of registered CeX customers.
As a precautionary measure CeX are emailing up to 2 million of their registered website customers who could potentially be affected. Customers, who do not receive an email, are not affected.
CeX, which also runs the webuy.com website, is asking customers to change their passwords immediately.
A good way to create a strong password is to use three random words and numbers, which are memorable to you, but not for other people to guess.
Managing director David Mullins said: "This was a sophisticated breach of security and we are working closely with the relevant authorities to help establish who was responsible. Our cybersecurity specialists have already put in place additional advanced measures to fix the problem and prevent this from happening again”.
Report fraud and cyber crime and receive a police crime reference number.