bro psu's are the most important part of any pc, psu hold the life of every other component dont cheap on it buy 80+ Gold and above prefer Superflower/seasonic or Corsair
When you buy a used psu you are overall getting less lifetime out of it since it was used, plus it could have defects and dust build up, buying a new psu ensures that it has maximum lifetime, no problems, and potentially longer lifetime since you can prevent things like dust buildup in the first place