Introduction
Buying a used musical instrument can be a good way to save money, but you need to be careful. Here is a checklist.
Guitar
Check the neck - is it straight, do the neck coils work. Look for scratches, cracks, loose joints. Try all the buttons and switches. If an electric guitar, connect to an amplifier - do all the sounds work, is there any noise. Check the languages and mechanics.
Piano and digital piano
Acoustic - check if all keys work, if the sound is consistent, if the pedals work. Have a tuner check it out before you buy.
Digital Piano - Check all keys, functions, pedals, connections, speakers.
Wind instruments
Check the case - dents, cracks. Valves (woodwind instruments) - do they close tightly, are the pads worn. Sliders (brass instruments) - do they move smoothly. Wash and disinfect before use.
Drums
Check the drum skins for wear, dents. Frames - Cracks. Hardware - bolts, screws. Try the sound.
General recommendations
Try the pill before you buy. Ask about the history - how old, how much used, whether it has been repaired. Ask for original documents, bags, accessories. Compare prices. If in doubt, take a specialist with you.