If I were asked for W-2s, I would respond by asking for the same information from the company on the existing sales reps.
For sales people, it might be what they made in terms of compensation plan and customer loyalty in terms of what they can bring to the new company. One thing not mentioned in the article, but which I've witnessed, is that great sales people often bring their customers with them to a new company if they can. One recent example from people I know involved a guy who moved to a competitor with literally over a million in sales. He just fed his customers's egos to get and keep their acquisitions then benefited when they sold their own companies on the change that mainly benefited him. Made a killing on this approach despite not even trying hard to get new customers lol. So, always worth remembering that this might happen if your product/service is interchangeable with others.