Couple of rare ones (one more than other).
Voigtländer Color-Ultron 50mm F1.8 M42 mount
Cosmetics is 9/10, with minor scuffs. Mechanically and optically 9/10. Comes with third party caps. $220 OBO.
Carl Zeiss Ultron 50m F1.8 M42 mount
This is the rare precursor to the above, with the unusual concave first element. These in M42 are going for 450-500USD on Ebay, but this one's priced cheap for the following two reasons:
1. The aperture blades have some rust or some anomaly on them so that the auto aperture pin doesn't quite function as you'd want it to. Specifically, it will stop down fine, but won't "spring" back open on its own. This means that if you're going to mount this on an M42 film camera, the shot will come out fine, BUT, you will have to manually adjust the aperture ring to wide open to open up the blades before your next shot. If you're like me and plan to use this on a DSLR with an M42 adapter THAT HAS A LIP to constantly depress the auto aperture pin, then there's no problem at all, since you'll be doing stop-down metering anyway.
2. There was some auto-aperture related mechanism that I removed (in the rear element shot, you can see the crescent shaped hole) in order to permanently set this to "Manual" mode (since this lens doesn't come with a switch to change between Auto and Manual modes). I do believe I have the parts in a bag somewhere, but I'm not going through the trouble of reassembling it; it works great on a DSLR as is, and you'd only want to reassemble if you're going to use this on an M42 film camera.
Besides the above two mechanical caveats, the optics are great (9/10), cosmetics fine (8.5/10). $350 OBO
You pay shipping and PP fees.