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.