Re: A.H. Fox 12ga Sterlingworth
From the pictures you have posted, to me it looks like the receiver, and probably the barrels, have been re-blued. The receiver was originally case colored and NOT blued.
Wood looks refinished too.
$850 is NOT a reasonable price if my assessment is accurate.
If the barrels have been hot blued, and not rust blued, the solder that holds both barrels and top and bottom rib together may be compromised and may actually loosen and begin to separate either with just age but may be accelerated with shooting.
$300 - $400 would be more realistic if everything works correctly now with no cracks, splits, barrel problems, etc. But don't be surprised if barrel problems occur later on. Sometimes they do, sometimes they don't.
You pays your money, you take your chances. Better to find something that has no "issues".
Hope this helps and hope you get some other opinions.