A qualified appraiser has formal education in appraisal theory, principles, procedures, ethics, and law. The appraiser should be up to date on the latest appraisal standards. Continuing education and testing are the only ways to ensure this competence.
The appraiser you hire should be familiar with the type of property you want appraised and know how to value it correctly.
Expertise on a particular type of property is not enough if the "expert" does not know how to evaluate an item for its appropriate worth. Without appraisal training, these "experts" have no way of understanding the complicated variety of marketplace definitions that are used to determine appropriate values for appropriate uses.
For example, a museum curator may be able to authenticate a work of art, or a jeweler may be able to determine the identity of a gemstone, but neither may be able to value those items correctly unless they follow appropriate appraisal principles and procedures.