Password trading is a well known way to share accounts, for example if you have a compromised account they will post the user/password on a forum where all the others will be able to find and login to your system.
To prevent this from happening jCore comes with a password trading protection which enables you to notice accounts compromised by password traders by verifying the ips the account is used from and if there are more then 3 different ips logged with the same account at the same time it's pretty much sore that one can't be at two places at the same time not to mention 3 so when there are 3 simultanious logins with the same account but with different ips the system automatically kicks them out and closes the account for 60 minutes while sending you a notification email about the situation this way letting you know that something is wrong and you should have a look on things.
From the admin Global Settings area you will be able to customize these settings change the number of allowed simultanious logins and opt out from the notification emails which is not recommended but just in case.
With the password trading protection you should be able to recognize compromised accounts and make the neccessary steps to protect these accounts like changing passwords.