Enabling Player Checks is a great way to prevent fraud on your store and will allow you to verify if someone has logged into your server (And using the same IP address) before being able to continue with a purchase.
Go to Fraud > Player Checks.
Configure the options as listed below to ensure the feature works as you expect:
Enable this option to ensure that the player has accessed your server before they can make a purchase on your webstore.
This is the amount of time which a player has had to of logged into your server before they can make a purchase. We recommend setting this to a value of around 3 days.
Enable this option to require the player to access your webstore with the same IP address as they used to login to your server with.