Is this a private seller or a dealer?
Have you had your independent piano tech evaluate the instrument? Such person can inform you on the condition of the piano and also their opinion on a reasonable price.
Look at the serial number, there should be a letter before it. This letter is the version / revision of the C3 model. There have been design changes between versions.
I have a few days to decide, since there might be others interested as well 
This may be a selling strategy. Make sure you have a counter offer prepared with a date of your own. A tech's evaluation can help you determine your offer. Be prepared to "show" no interest and walk away, especially if you have found a connection with the instrument.
Anyway, I'd avoid pre-1985 Japanese pianos. Their built for the climate in Japan, and are not suitable for the US.
Do you have a source for this? That is a pretty big claim.
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My personal take on used Yamaha C3's: they can be excellent instruments if voiced and regulated well. IMO a well prepped, broken-in used C3 can outshine a brand new stock C3. This opinion may come with a little bias as I have a used C3, that does not carry the typical Yamaha sound found on new instruments. With a good tech, it is possible to bring the sound closer to an S series.