I would think most folks that know the difference would buy your box frame disc over a new angle frame disc. That would put it in the $750+ price category around my part of the world - if it still looks like the pictures of a few years ago.
A new box frame disc that size is going to cost $1200 for a low cost model....and several hundred more for a name brand. Still....it isn't new.
Based on the above.....If I was selling it, I'd ask for $795. or, mabye $895. and post it on CL.
I'd give 800 for it if you were closer.
