It is possible to buy relatively cheap used components on Ebay (condensers, objectives, etc) but I have seen that some of them were extraordinarily expensive. The reason is, that some components are not manufactured anymore and that the demand for some rare second hand components has greatly increased.
I recently wanted to buy a projection ocular for my microscope to camera adapter. These are not made anymore, because the company changed systems and the projection ocular is not compatible with the new system anymore. The cost of this special ocular was higher than the price of a complete new microscope. Universities and research organizations have the cash to pay for these, when they need these components. This drives the price up as well.
One should not forget that microscopes can remain useful for many years and that in the meantime some rare components might start to disappear from the market. With the introduction of infinity-optics, the interchangability of parts between different manufacturers has increased yet further, making it more difficult to find useful second hand components.