While Forney is still in business, their welders, I believe, are all sourced from other manufacturers. Without a picture of your unit, I'm unsure as to what type of welder you even have. Suspect that it is probably a single phase 220v AC only unit as that was the majority of Forney's product line back when they were in production. The company became what it was/is by supplying farmers with an easy, basic welder to use on their new REA electric lines. Back in the days when they were new, the Forney line was a cheap alternative to Miller, Lincoln or Hobart.
If indeed your unit is one of Forney's old AC only machines, a $100-$150 would be more in line as a starting price. AC/DC and it could probably reach $200-$250. I'm not trying to grind you down on price as I'm not remotely interested in purchasing said welder. Just tossing some figures out there for you. Matter of fact, I'd quote the same price range for most old Lincoln, Hobart or Miller welders of that vintage. Best of luck on selling the machine.
Miller 251, Lincoln PrecisionTig 275, Miller DialArc 250 AC/DC, Hypertherm 900, Bridgeport J-head, Jet 14" lathe, South Bend 9" lathe, Hossfeld bender with a collection of dies driving me to the poorhouse, Logan shaper, Ellis 3000 bandsaw, Royersford drill press and a Victor Journeyman O/A.