Did he specify what component of the 1st stage made it "not diveable" and is this a part that is not available? Is it a case of the repair will cost you more than a new 1st stage? You have provided very limited info (old SP reg). 1st stages are relatively simple. I'm still diving a circa 1980's Sherwoodl and I have friends diving even older SP and aqualung stuff. Just replace what isn't working unless the cost to do so is more than it is worth to you. Are you sure the issue isn't that they are upset that you purchased from Craigslist?
He did not...but I can ask him what specifically was wrong and how much it would cost to get replacement parts.
All i know is it's a SP 1st stage that doesn't "depth adjust" (per Tech). Shop probably doesn't appreciate the craigslist purchasing, but on a low budget you can find some deals. I just got into diving last October coming up on 100 dives so I'm still a newbie.