The age is not a problem at all. Steel tanks are often seen in service from the 1950's for scuba diving and someone was mentioning they got a commercial steel gas cylinder from the early 1900's once.
I would try to convince they guy they are worth much less, but $50 is still a good deal as long as they pass hydro, or I would return the tanks and expect my money back, arrange before the sale.
At this point PST is out of business, and their DOT number is invalid. So far no one has ever had anything to do with DOT and scuba tanks related to PST. At least not that I have heard or seen written somewhere.
Huh, What are you talking about? And as I said the DOT exemption is only for the HP tanks and even those can be renewed by an individual. There was a thread with the procedure or link to it. Or go over to the deco stop and ask there.