A problem when one is looking for spare parts on eBay (and other online sites.)
I am in favor of self servicing. When one gets to the case stuff, that is a different skill set. For the most part this is not an issue with the pre 1970s stuff. Once upon a time, there were a large number of independent watchmakers. Most jewelers had one as did some drugstores (dry goods.) There were a lot of material houses and lots of spare parts. In some ways the markets were over saturated.
If one is lucky there is someone selling such NOS parts in the factory or distributor packaging. This supply is dwindling down. There also seems to be a practice of opening the packaging and selling the individual pieces. The hardest parts to find are consumables like gaskets, crystals and bezels. Gaskets have a shelf life which makes it hard to know if one online is fresh, or something pulled from another watch. Or sat around for 40 or 50 years.
Ideally there should be some sort of program or certification for those of us who self service our own stuff. It is not economical (Mostly liability.) to work on other folks watches. Customers have high expectations.
In practice there were too many taking the easy out and manufacturing their own watches from spare parts. So one can see why the manufacturers want to limit the parts availability. This also seem to be more of a liability issue. The temptation to make something from nothing happens even at the high levels.
Such seems to be human nature. To game the system. We all want that barn find. Some of us to simply enjoy it. Others feel the need to profit by it. Rules these days seem more like guidelines, which is probably the cost of freedom.