Are you licensed?
While a full, legislative-based regulatory system exists to monitor real estate appraisers, no such system exists for personal property appraisers. You should seek an educated and experienced appraiser who belongs to one of the top three appraisal organizations: ASA, AAA, and/or ISA.
Further, you want to be on the lookout for an appraiser who offers to appraise and then buy or sell your items. Reputable appraisers are disinterested parties whose only goal is to provide an objective analysis and conclusion of value.