Without an array of photos, it's hard to assess the value of a tractor. I know that finding decent 150's is getting more difficult but this one better be in mint condition to command $600.00.
Sometimes, people pay waaaaaaaaaaaaaaaaay too much for a project tractor, lose interest in it and are then surprised when no one is willing to pay anywhere close to what they paid a couple years earlier for the tractor.
I bought a 150.. It's got straight tin, needs a gas tank and it cost me $100.00.