Another thing... I noticed in the pics of the other ads you posted that these compressors were in a commercial environment where there was a possibility of employees present (evidenced by forklift, many pallets, etc). If this is the case, then it is highly likely that at some point or another, an OSHA inspector came in and inspected the tank.
Given the age of the labels on the tanks in the pictures that you showed, the probability that the tanks were at some point inspected by OSHA is even higher. This applies to California and metropolitan areas of New York, the two most progressive (some would say aggressive) coastal states with highest population densities where regulations (some would say invasions) are on the cutting edge.
So, if CAL-OSHA inspected those tanks, they would have stamped (with a punch) a serial number on the head of the tank. The numbers would be at least 1 inch in font size. When you look at the tank, look for that serial number, and write it down. Then, go out to your truck and "think about it" for a while, and while you are thinking, call the OSHA office in So Cal that is responsible for keeping track of state inspections for pressure vessels, and have the record pulled for the serial number you obtained.
They can tell you on the spot if the tank was condemned for use in commercial service with employees present. They will have made that determination utilizing an ultrasonic thickness tester, comparing their readings with the minimum thickness specifications found on the welded on National Board Registration tag that is part of every code welded ASME pressure vessel, which all the Husky's in your links have.
This way, you don't get burned by a business craigslisting away a condemned tank they can't use in their shop anymore. Also, look for any circled X punched into the tank. That is a sure sign of state condemnation.
