The second-hand mobile phone I bought has just been used. There is no account lock, there is an account on the back.

For the second-hand mobile phone you bought, there is an account lock when you first use it or there is an account in the background. The possible reasons are as follows:

1. Device reset is not performed correctly: the seller of second-hand mobile phones should perform the operation of restoring factory settings before selling, so as to clear all user data and account information. If the seller does not perform this operation correctly, the buyer may encounter the problem that the account is locked or the account is still in the background when using the mobile phone.

2. The account has not been completely cancelled: the second-hand mobile phone may have been sold by the previous user without completely canceling the account. This means that the buyer will encounter the situation that the account is locked or the account still exists in the background when using the mobile phone.

3. Theft or illegal operation: Sometimes the second-hand mobile phone may be stolen or illegally obtained, and the seller has no legal account information. In this case, the buyer may find that the account is locked or the account still exists in the background when using the mobile phone.

In this case, the buyer is advised to take the following steps:

1. Try to contact the seller: If the seller is honest, he may provide a way to unlock or cancel the account. Buyers can communicate with sellers and ask for help to solve problems.

2. Reset the device: The buyer can try to reset the device and clear all user data and account information. This can usually be done by finding the option to restore the factory settings in the phone settings. Please note that resetting the device will delete all data on the phone. Please back up important information in advance.

3. Seek professional help: If all the above methods fail, the buyer can consult the mobile phone manufacturer or professional maintenance personnel. They may provide more specific solutions, and may also help solve the problem of account locking or background accounts.

Generally speaking, when buying a second-hand mobile phone, the account may be locked or there is an account in the background. It may be that the seller reset the device incorrectly or failed to cancel the account, or the mobile phone may be illegal. Buyers should take corresponding measures to solve the problem and ensure the normal use of mobile phones.