same user role or copy the user role to be same as the other role

Contributor has the following capabilities

• delete_posts
• edit_posts
• read

You can create the “Seller” role to have same capabilities as Contributor

add_role(
    'seller',
    __( 'Seller' ),
    array(
        'read'         => true,
        'edit_posts'   => true,
        'delete_posts' => true,
    )
);

Warning from codex

NB: This setting is saved to the database (in table wp_options, field wp_user_roles), so it might be better to run this on theme/plugin activation

In functions.php, you should use this in after_setup_theme hook.

EDIT

You can also create a user role who have same permission as another this way

add_role(
    'specialuser',
    __('Special User'),
    get_role('seller')->capabilities
);

It will grant specialuser same capabilities as seller.