If you want to save money, buying used anything can be a good way to go. This is no different when it comes to digital pianos. You can buy a good, used digital piano if you know what to look for. But there are certain pitfalls you will want to avoid. Regardless of how cheap the digital piano is, it's not a good deal if it doesn't play right.
Check the key action - Play each of the 88 keys one at a time. Do they all still feel good? You want to see if any of the keys have a tendency to be unnaturally stiff when pressing down or if any have a tendency to stick down and not return to the resting position after being depressed. You will also see if any keys wobble from side to side by going through this exercise.
Check if the touch response still works - Touch response is the digital piano feature that varies the volume based on how hard you strike the key. This feature gives you more control over the piano's sound, and it brings you closer to the feel of an acoustic piano. To check the touch response requires playing each key again. And don't just play each key once. Play it at all volumes, from striking it softly to striking it very hard. I'm not talking about adjusting the volume knob to get this volume change. I'm talking about adjusting the velocity with which you strike the key. You may find that some keys have lost their touch response and always sound loud no matter how soft you play them. You wouldn't notice this unless you played each note individually. A side benefit of this is that by playing each key you may notice that some keys stick when played at different velocities, whereas in Step 1 they hadn't stuck.
Check the pedals - Make sure the foot pedals are all in working condition. The most important foot pedal that needs to work is the sustain pedal (the far right pedal). But by all means check the other two pedals if it has them. If the digital piano only has one foot pedal, it will be the sustain pedal. Play each note with the sustain pedal held. Release the pedal and play the next note.
Turn up the volume - Turn the volume up as loud as it will go. You want to see if the speakers crackle at higher volumes, or if they crackle at any volume level in between. Definitely look for crackles with the volume up while playing the lower notes.
Turn down the volume - It's not a bad idea to see what the digital piano sounds like when the speakers are off. You can hear squeaks and other noises that the volume would have covered up. You may be able to hear odd noises, such as clanking, that may indicate there are some mechanical problems.
Check for any damage - This is easy enough. Just look at it. What kind of shape is it in? What kind of damage has the body been through? The more wear and tear the body has been through, the more likely something internal may not be working (or soon to not be working in the future). Here's a tip: Check the keyboard for any stains from drink spills. You probably won't see any because the seller would clean this up. So press down each key and look for stains on the sides of each key. The seller is less likely to clean this up, and it will let you know something was spilled on the keys.
Those six steps should help you avoid getting a damaged used digital piano. But don't forget to use your judgment with the seller. There are dishonest people out there, and if you think you are dealing with one I'd suggest back out. Especially if the deal seems too good to be true.
