Who is Arbogast from Trouble Is My Business and what is their importance?

John D. Arbogast is the expert Anna Halsey hires to authenticate a selection of photostats taken of fifty thousand dollars worth of notes that Mr. Jeeter's son gave to Marty Estel. He is described as incredibly overweight, with a face that resembles a pink basketball. His hair is grey, cut short, and his suit is in need of a good cleaning.