Good points.  I actually bought this keyboard second hand at a Guitar Center 10 or more years ago and was only using it to enter MIDI drum data at the time.  I didn't even notice the problem till several years later when I started using it as a real keyboard.

If you bought this off ebay I'd email the seller and tell them the problem and ask if something can be worked out. Most the time if it's an honest seller they will give you some sort of refund. Or you can out right tell then what you think it is worth and ask they put the money back in your paypal account. Also if you paid with paypal you may be entitled to a complete refund including shipping through buyer protection plan. 

If you are considering buying a used keyboard, be sure to actually test each of the keys to be sure they produce a sound and that the velocity is correct.  I bought a used Roland A-33 keyboard and didn't realize that the third F# had no velocity; the note works but is always full volume. 

I'm pretty sure it can be fixed but, with defective keys, a keyboard is worth a fraction of a fully functional instrument.  Depending on exactly which keys are bad, how they are bad (no sound at all, too soft or too loud) and what musical keys you work in, the instrument might still be usable but you should not be paying much for it.

The Roland A-33 has no built in sound (it's just MIDI Keyboard Controller) so the test required something to test it with.  In my case, it was an old Yamaha QY20.  A quick run through the notes would have revealed the problem had I done it.  If you're buying through ebay, make sure the seller confirms that all keys work.