I have several good quality ATX computer power supplies (Corsair, Thermaltake, etc) that I reviewed for a website over five years ago that I want to sell on ebay. My question is after sitting in their boxes for over five years and having been initially used for only a couple of hours are there any possible problems? I suppose it's the electrolytic capacitors that concern me the most. Unfounded?
All materials have a half life and the entire supply will eventually decay into a block of iron, even elements lighter than iron, apparently through a different process.
While it is hard to give a direct answer I have PSUs laying around here I tested for reviews about 20 years ago. They all work just fine. I also had an old Cooler Master PSU which failed miserably after a year of service on a relatively light load so go figure. In your case, short of a crystal ball, I would offer a buyer a 6 month (or number of your choosing based on asking price) money return guarantee.