calculated field in views based on another view
One way to do this would be to create a field template for that specific field.  Then you could first print the value of the seller price, and then do the calculations in php and print the difference.

You can specify that the field template only be used for a specific field in a specific display of a specific view by naming it correctly.  You'll find the pertinent names in your view edit page under Advanced -> OTHER -> Theme: Information.  The default contents of a template file are also available there by clicking on the field name.