Painted or Galv?
Pressure?
Manufacturer?
Boot?
Covered with stickers or reasonably clean?
Add $35 for a DIN, add $40 for a ProValve.
A Galv, PST, with a nice Thermo Valve, a boot that isn't cracked and all broken, and not all covered with "divers do it deeper" crapola would probably fetch $50 (low end) to $75 for a very clean one.
Log onto eBay, and do a search in Tanks. Search for "completed items" and it will give you an excellent idea of the market value of stuff.
Also depends on where you live. In SoCal, there is a movement away from smaller steel tanks and into larger tanks, so the 72's aren't moving real well here. However, I just sold one last week for $75 that was pristine - PST, Galv, no stickers, nice boot with a Thermo Yoke.
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Ken