The issue of appraising telescopes and related equipment comes up often among members of the Antique Telescope Society, many of whom are collectors. Some years back, we had a professional appraiser speak at one of our annual meetings. Surprising many of us, she warned us regarding the legal danger of offering appraisals without being credentialed as a professional appraiser.
Astronomy magazine subscribers can read the full answer for free. Just make sure you're registered with the website.