Hard Ball,
The 520 was originally ordered by the New York State Police around 1980. The order was never completed, and non were ever delivered, so I don't think there are any that are NYSP marked, which would indicate that the NYSP had taken possession of the gun.
This was the only S&W N-frame .357 ever made without adjustable sights.
According to Jinks & Nahaus, there were approximately 3,000 made, so it is not particularly collectible -- more so because the box is there, and the condition.
If all the original paperwork and any tools are there, it would probably be priced right around $500 or a bit more in Virginia.
How much is the dealer asking?