Indeed, it’s a lottery.
The only big purchase second hand cartridge I bought was the Io Ltd and I had it confirmed by ANUK that they had rebuilt it three months earlier (thanks to @Ciderpig for getting that info for me) Even so, I was a bit nervous until I received it and heard it playing. Seller based in Prague and no idea who he was. What could have possibly gone wrong!
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I’m compelled to take gut wrenching acid inducing punts on interesting stuff I in No way need. Cartridges are an interesting penchant.
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Some people tend to compare Koetsu Black with Denon 103, meaning it is dependable on a context of use. You put it in an inappropriate tonearm and you fuck the loading - you get a crappy sound. You do things correct and it will shine. I have both versions, old with black plate and new, Goldline with golden plate, both overhauled absolutely up to the task by one of the very few remained Koetsu specialists in this world. They are different, the older is more lush and in the line with any Koetsu prejudice, the new is just a tad more get up and go, energetic and dynamic. It’s a matter of taste but they are both very good for what they cost me - a donor unit plus work etc - around a 1000 or so, which is 2-2.5x less than the new stock. In fact I like them both so I avoid using them, apart from the time I gave them to have them properly run in, say 100-120 hours. Understandable since I use another cartridge from rather similar school of sound - SPU Spirit.
For me this is the way to go if you want Koetsu Black.
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Out of interest, what is your preferred loading for the black?
So it’s shit then?
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40 ohm input on Tango MCT-999 SUT. Also works decent into Ortofon T-5000 which I think it is 10 ohm.
Currently down the Koetsu / SUT hole.
I have a Koetsu MCR-1 SUT
Spec:
General characteristics
Frequency response 30 Hz - 50 kHz
Gain 26 dB
Input resistance 10 kΩ
Ratio is therefor 1:20 (26 dB)
Running it into a standard MM stage (Presents 47 KOhms.) with a gentleman’s dose of mathematical assistance (Thanks Guy) this means the cartridge will see 47000 / 20 squared ie 47000/400 or 117.5 Ohms when using Koetsus own SUT.
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Even then it’s tricky. If Koetsu are implying that the secondary of this (their) transformer be loaded by 10 KOhm (hardly a standard load presented by a phono stage) then the implication is that the cartridge should see 10000/400 ie 25 Ohms. I think that’d be too low.
The darkest of arts
So does this mean that without additional loading very few cartridges are seeing the required load from a SUT/MM phono?
Generally, yes.
I found this helpful right up to the point where my head exploded.
http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html
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Another reason why it can be easier to sort these things with a combo of educated guesswork and ear sometimes 
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Searching online it appears that Koetsus typically have an internal impedance of 5 Ohms and. according to the specs, want to see a load of 30 Ohms. They also give an output of 0.4 - 0.6 mV.
Assuming a typical mm stage with an input impedance of 47 Kohm a straight transformer that presented the cartridge with a 30 Ohm load would need to have a ratio of 1:40. All very well but such a ratio would result in a hefty output of 16-24 mV which might give some mm stages difficulties
The way round this conundrum might be to use a lower ratio transformer, say 1:12 or 1:15 and then load down the secondary such that the load presented to the cartridge is the desired 30 Ohms.
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A few beers + 
= talking voodoo
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Or set the load on the primary. Depending on the transformer this might be preferable.
Dunno but I just set my SS MC stage to 100 Ohms and it sounded ok.
Hoping to have the Koetsu SUT back soon (It’s been on gap year(s)) I’m curious to measurebate and note what it is actually doing. I’ve not found it possible to hit 117 Ohms with the AN S7C