Show menu only when user ticks the checkbox
Go to admin/config/people/accounts/fields and add a boolean field.
In your MYMODULE_form_alter() add a '#type' =&gt; 'checkbox' form field and a custom submit handler.
Inside that custom submit handler save the checkbox value into the user's boolean field from step 1.
Inside your MYMODULE_menu_alter() call global $user and check that field's value $user-&gt;field_sell_themes and build the access control accordingly.