They are representing the computer as a 2007 machine, but they don't know much about it... The serial number is XB31600
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They are lyi^H^H^H misrepresenting the machine.
For sure, if it's a G4 it wasn't made in 2007.
To decode the serial number the first two characters (in this case XB) indicate the factory, the third character (3) is the year - in this case 2003, and the fourth and fifth characters (16) indicate the week number. Therefore this machine was manufactured in April 2003. It isn't worth more than a few bucks (literally).