You're right. A properly repaired or restored FM-3 tuner is a fantastic tuner for the price.
The sensitivity and selectivity is quite poor compared to good solid state tuners from the 1980s but the FM-3 sounds terrific on strong stations and in itself is quite capable. The thing to remember is that the FM-3 needs to be in perfect condition and in perfect alignment to sound good. That means that most of the time it will need to be repaired at minimum, most often refurbished and many times completely rebuilt from scratch before it performs well.
That ebay listing appears to look like a nice example of a factory built FM-3, but the pictures on the ebay listing are dreadful and blurry, and there is very little explanatory text.
The problem is that even totally restored FM-3s have trouble selling over $200, so why pay $329 on a tuner that you will no doubt have to repair to get working correctly?
Better to get one that is less money and invest the rest of the money you were going to spend to rebuild it properly.
Oh and to answer your question on the availability of parts: You can get most parts except the critical ones. The EMM801 magic eye tube is extremely rare. The transformer is extremely hard to find. The tuning capacitor is impossible to find. The germanium diodes in the multiplex decoder are getting difficult to find. Most of the other parts (or their equivalents) are available. You can find all the replacement tubes easily (except the EMM801)
