Mario, Can you ask the seller to give you a serial number of the piano?

Here's some info you should read before purchasing a used Yamaha piano:
http://www.yamaha.com/ycaservice/group004/grayacoustic.htm

And to check the age of the piano: http://www.yamaha.com/ycaservice/group004/ageyamahapiano.htm

Good luck!
..rb