There is no such thing as an "extractor rod" on an HK pistol.
I imagine whoever said this is instead referring to the extractor, which on an HK is external to the slide and long. I suppose it could be misconstrued by someone as being a rod.
The steel HK uses for this part is very hard and it is difficult for the black phosphate coating to adhere to it. As such, it naturally "patinas" (develops a purplish or golden hue) with age. This type of blemish is completely normal and purely cosmetic. The Hostile Environment treatment, which is a type of nitride finishing, will still protect the metal from corrosion.
$625 would be a good price provided the pistol was in otherwise decent condition. Especially if it was a .45, as they seem to command a slightly higher price tag then the .40 or 9mm models.
"Don't hit at all if it is honorably possible to avoid hitting; but never hit soft." ~ Theodore Roosevelt