Hi Sandy,
Having bought a 2nd hand,
Yamaha kb, l can recommend a few checks. There are others here at this forum who will be more thorough. . But here is my list of checks . .
*Play every single key black and white. Make sure they all work. Listen to hear if any are louder or softer than the rest. Check them all at low and high volume.
*Check that the volume knob and all the knobs and slides work smoothly and efficiently without static.
*Try out a few of the accompiament features that they work. Try a few different sounds piano, organ, trumpet, flute, . . Happy?
*manual included?
*sheet music holder ok?
*Switch it on try some things. Switch it off . . Switch it on again. Is it normal? it should be just like a standard piano. Now try some styles and voices and chords. All good? Happy?
* is the screen display working? Not scratched?
They are a few items to check off the cuff.
What do you mean refurbished?