My 2 cents. I think the agent is more important than the company he works for. Remember commercial agents specialize more than residential. If you live in the area, interview a few. If not, ask the Realtor board for the top producers in the area over a few years. A very large deal can skew the numbers over the short term. After narrowing the list, do some interviewing, get a list price suggestion and the reasons why and how.
This is for listing. For a purchase, I would ask for recommendations, narrow the list, then interview. Remember that unless specified, the agent works for the seller. Get a buyers agent, and be sure that any contract so specifies.