I have had brilliant results from both the LTM Jupiter 12 I bought new in the 1970s as a camera mad schoolboy and the 1988 'new old stock' Kiev/Contax fit J12 I bought in January this years as... well, many people would say simply someone who is simply mad. Happily, the second J12 came from a 'proper' eBay company that otherwise specializes in medium-format Kiev stuff (Arax).
I have just bought a third: 1961 silver-finish one with the protective shroud around that frighteningly vulnerable rear element. I've not tested it yet, but fingers crossed...
As others have said, if you are unable to inspect the lens you are buying, one clue that all might not be well is the finish - un-messed around with silver FSU lenses will either show the odd trace of corrosion or yellowed protective lacquer. It's when you seen one that has random-direction scratches from being attacked with a Scotchbrite or Brillo pad that the alarm bells should ring - or the asking price should be especially low.
It is great when sellers post test images - the chap I bought my Jupiter 9 from did just this and it works just as well as I'd hoped. (The other great thing here, at least for those of us who persist with film and Kievs and Contaxes, is that the asking price for an excellent J9 in Kiev/Contax mount seems to be about half that for an LTM version in the same condition.)
That said, I would imagine I've been lucky so far, because you cannot see inside these things.
