How do you go about getting an honest gun appraisal for either buying or selling a used gun?

The Blue Book does appraisals. The fee is $20 per gun. I suggest you read their policy on this. It is on page 68 of the 18th edition of the Blue Book.

You might want to contact your insurance company and ask them about firearms appraisals and for local references.
Blue Book of Gun Values and Standard Catalog of Firearms are the two standards. BB uses percentage of finish, Std Cat uses NRA condition guidelines.

Flaydermans is the absolute best for it's somewhat limited field - antique American arms.

A new reference, and quite useful, especially for oddball stuff, is RL Wilson's Official Guide.

The tough part of any appraisal is evaluating condition. This includes an assessment of any modifications or refinishing (both of which reduce collector value). As you will see from looking at the price guides, conditon can quickly double or halve the value of any gun. This cannot be done remotely -- the appraiser must do a "hands on" inspection.

As you noted, it is difficult to find an unbiased appraiser. Anyone who truly has enough knowledge to accurately appraise firearms is inevitably either directly or indirectly involved in the buying & selling of firearms. I wouldn't give much credibility to a general insurance appraiser's opinion of a gun's value.