There are Several things that you should check before buying a used monitor, These are the things you must know before buying a Used monitor.
1. Check All the Buttons, Make sure each and every button works fine don’t be shy to check that multiple times.
2. Try switching Monitor on and off and see if the Screen has vibrations or not, Not having vibration is a good sign.
3. Place a Mobile beside Monitor and see how much the screen is getting affected, minor vibrations are ignorable and if its more then its well used.
4. Smack the Monitor once or twice, it may sound weird but that was the reason why i bought a new monitor, My monitor would work all good if its smacked good, check that you are not getting my monitor, because its used insanely and there won’t be much left for you to use 
5. Check for the details of Each colour.
6. Switch on the Monitor after unplugging the cable from CPU, check if the No Signal Message is displayed.
