This is not actual a very difficult question, although I personally still don't know what the answer to this is. I was not asked any difficult or unexpected questions so this is the "most difficult" one I can think of.
I had first answered this question using my basic knowledge of what I consider sales to involve. I mentioned the importance of long-term relationship building, competitiveness, and having a goal oriented nature. However, the feedback I received strengthened my own definition further and emphasized the use of collaborative selling as the most important technique. Not only would this include relationship building and long term business development, it focuses on developing trust and respect between the salesperson and the customer.