treeline - 4:28 pm on Mar 19, 2013 (gmt 0)
If someone is hiring you as an SEO consultant you'll need a method to demonstrate your accomplishment. While actual sales are a beautiful metric, they aren't reliable for an SEO working with a business that has any other presence in the world. They'll work great for a website that is an island without other visibility.
Good press or an appearance on the front page of WidgetWorld Daily (unrelated to the SEO) could make you look great. Falling sales because of poor customer service, racist/sexist interviews by the site owner, or lack of inventory will make the SEO look incompetent.
An SEO Consultant needs to distinguish what their impact has been.