Not sure what to make of their worth.. If you're willing to live with some fairly old fashion pressure pot type conventional spray guns, and you have a use for it.... I'm thinking due to age $75 is about right.. Might try $50.
The one we had wasn't tremendously loud, sort of an old fashion "chugging" sound, I suspect it depends on what the speed of the motor is.
The handle is probably to lift if once its on the ground.. Its going to be fairly heavy. The one we had had a small tank under the cart it was on, with wheels. The "tank" on this one looks a bit home made. That pressure gage is a bit misleading. I don't believe the one we had could build much pressure above maybe 50 PSI.
Have you done a web search of the model number, etc? I suspect it was a fairly common unit back in the day..