Search your Secretary of State business archives online to verify that the company is legitimately registered as an estate sale business.
Ask for any appraisal certifications, auctioneering certifications or other related credentials. Then search the certifying organization to verify that they are still up-to-date.
Ask to see their current certificate of insurance.
Find out if the owner has a resale shop, booth or other such business. Knowing this allows you to ask about potential conflicts-of-interest with regard to marketing and fairly pricing your items.
Search their website for current and past projects, including client testimonials. Ask about a specific project and it's outcome.
Ask about their pricing philosophy and how they arrive at their asking prices.
Find out how they provide confirmation of sale totals or if they can provide transaction logs.
Ask about their procedures regarding the security of your property during sale operations.
Ask for examples of when their business ethics came in to play and how they handled the situation.
Make sure you get a written contract signed by both parties before any work begins!