You're lucky to find one in good condition for that price. I looked for a long time for a good used 84 inch with a hinged back blade. Finally had to go for a new KingKutter with a hinged back blade for $749.00.
How do you like the hinged back blade? I'm kind of stuck on the question.
The KK Pro Hinged Back on EA.com seems to be one of the heaviest box blades that they sell. I want to go with a 5' one because i'll be using it for trails in the woods, and this one comes in at 520 lbs:
King Kutter Professional Hinged Back Box Blade
The problem is that it comes at a premium price...$880 (shipped). I could buy a cheaper one for a couple hundred less, but am going to be at less weight, and no hinges. Given my uses (lots of driveway grading and work in softer soil), i think i need both..weight and a blade that doesn't load up.