However, professionals can be found online. For historical instruments, one well-known specialist business is Tesseract, www.etesseract.com, run by David and Yola Coffeen since 1982. They deal in early scientific and medical instruments. For years, they have published Rittenhouse, the journal of the historical American scientific instrument enterprise.
But when I myself am wondering about the current market value of astronomical equipment, I turn to the Astromart website. I can search its vast archive of sales by equipment model name or by any other keyword. In that way, I’m able to document recent prices paid for almost any imaginable astronomical hardware.
Antique Telescope Society
Magdalena, New Mexico