I recently came across the excellent article from the New York Times wealth management archive. My takeaway: choose an appraiser who has extensive expertise in your specific property! This is particularly true if the item has significant value. While any appraiser who is properly trained and certified can produce a credible report with the proper format and cataloguing, values could vary significantly (either high or low) if the specific expertise in the subject property is lacking.
The Specialized Art of the Appraisal- NY Times