Did you get a look at the tire? I've often noticed NOS rubber seems okay. No idea why, just guessing: maybe it's the mold release? Or not being stressed (not inflated)? Stored indoors?? Maybe a rubber chemist can help explain it?!?
If you looked at the tire and it was dry rotting, then of course don't buy it! I'd flex the tread to open up any dry rot cracks that might be there so I can see them, if they're there. I suppose in this case you could feel outraged if you like. But I'd be inclined to assume the help might not know as much about NOS rubber as you do. Maybe they need to be informed...
'09 Trek 7.3 FX hybrid / Jett 155mm
'09 Cervelo P3 TT / looking
'11 Cervelo S3 road / Selle Royal Seta 155mm
Ischial tuberosities: 140mm center to center