Estimates of value online can be meaningful, bearing in mind that most
accordions are going to need a "reed overhaul" after at most 50 years
and might need tuning if rust or tobacco smoke etc. got onto the reeds.
Needing tuning is a problem because there is no easy way to do that.
Filing the reeds causes uneven response. The best way to tune a reed is
use sandpaper or fine abrasives while protecting the reed plate. This
preserves the "reed profile" which is a variation in thickness, while at
the same time polishing the reed. I don't know any self-styled
technicians whom I would trust to work on my accordions, though I admit
I don't know of any who might exist since I live a sequestered
existence with my 100 or so vintage accordions, but experience, and
looking at the damage done by idiots on accordions, makes me think these
people are as rare in the country as the Sasquatch.
Any issues with the accordion are going to make the sale value a mere
fraction of what a new one of equivalent quality would be, bearing in
mind that equivalent quality is a chimera.
The community of people necessary to mass produce good accordions no
longer exists.