1) How much value do they place on organization? It takes an organized person to be a good buyer.
2) Ask about software skills on common platforms such as MS Office. Everyone can say they have them. Drill a little bit depending on how heavy your office uses these platforms. Do you need someone with Access skills? Then ask those questions.
3) Ask about their work style/work planning. How do they prioritize.
4) Work better independently or in a team? This is critical to how your office culture is.