guenii wrote:"does it have clean piano sound"?
An acoustic piano? No, not really - but since it's a synthesizer, you can probably coax a passable e-piano (as in a Rhodes/Wurli) type sound out of it.
And since it's monophonic - it means that you can play only one note at a time?
Yes. Which of course decreases its usability as far as piano-type applications go, but you should be able to get some bass & lead sounds from it. No idea about its worth - while it's somewhat rare, I don't think these command very high prices.