As a service tech I cringe when someone brings in an old school reg they got at a good deal.
The problem with ALL used gear is this: People get rid of dive gear because they dove it so much its time to get new gear, thus its seriously in need of love and attention or the trash OR People get rid of their dive gear because they don't dive it hardly at all and that sand and salt from the one trip they did three years ago is still eating away at the brass.
I don't recommend people buy used gear ever because the average new diver doesn't know what to look for when looking at used gear. Your LDS is there at the ready to supply you with brand new, rock solid, reliable, warrantied gear. Many of them will even sell used rental gear periodically which counts as used but you at least have someone to wag your finger at if their is an issue.
That being said the Conshelf 14 was a bombproof regulator and you see a similar design on the front of Kirby Morgans still to this day. Most service parts, diaphrams, seats, poppets, ect. are still readily available. Replacement parts aren't too difficult to find because everyone's got an old Aqualung/U.S. Divers for sell.
The best answer to give you is; I wouldn't buy it. There is no way for anyone here to gauge its reliability without seeing the reg. Add that you can get a brand new reg for under $400 that weighs half as much and there ya go...