A small disk will bring more than a wide one. There are ten buyers for a 8 foot disk and 1 for a 12 foot one. The guys down below that tell you a good disk will bring scrap price are dreaming. Any good 8 or 10 foot disk will bring good money. An IH or Solid frame JD can easily break $1000 and real sharp one can go to $1500.
Some three point disk sometime go cheap but they usually are completely worn out.
So if you found good disk for $500 I would grab it before it is sold to some one. If it is not selling then something is wrong with it. Make sure it has ball bearings and not spool bearings. The spool type work but if they are worn out replacing them can be difficult to find the parts.