What's The Parts Situation With Vintage Zenith Movements?

It depends on where you are and who you know. In the US, parts are as rare as the proverbial from a rocking horse. Independent watchmakers just can't get hold of them for love nor money. In the UK I can get hold of anything that is still produced through a certain parts supplier. Obsolete parts can still be obtained but only if you have a special relationship with Zenith, then they will make them for you as a special order. Luckily, a watchmaker that I use has such a relationship.

The ebay seller you are showing (I believe) bought a particularly nasty 135 a while back and has broken it for parts. Personally, I have no problem with replacing any part and still considering it to be original, other than a bridge with the serial number on it (or the main plate, if that was where the number was) Let's face it, you wouldn't scrap a vintage Ferrari because it needed a new crank shaft and you could source a NOS replacement.