The grease of the microscope gears might become solid with time. It is then time to send the microscope in for maintenance and re-lubrication.
Dust disturbs the image quality. It is better to avoid dust than to deal with it afterwards.
Use only those cleaning fluids that are recommended. Ideally the optics should be cleaned as rarely as possible.
Clean the optics as little as possible, as often as absolutely necessary. Every cleaning session introduces the possibility of damaging the optical surfaces of eyepiece and objective. Small dust grains may scratch the surface of the optics.
Microscopes generally do not require much maintenance. Keeping the microscope dust free is probably one of the most important maintenance tasks.
Unless you have worked with microscopes yourself and unless you know what you are doing, I would not buy them second hand.
During the past couple of years I’ve seen numerous microscopes that were is a desolate condition. Here are some of the problems that I observed. It may pay off to consider these problems when shopping for a used microscope.
A dirty microscope objective significantly reduces the resolution.
There are several accessories that are part of a microscopic workplace.
This article explains how the lamp of a microscope is exchanged. It may not apply to all microscope types.