If the nickle is damaged, polishing it won't help, and may actually cause more damage.
Try some Flitz, which I think, is milder than Simichrome but whatever, GENTLY. Rub too hard and you can rub right through the plating.
Polishing will not help actual scratches or chips in the plating.
Having the gun refinished, even by Colt will cost, and you will likely wind up losing money on the deal.
Its very unethical to have a gun refinished, then claim, or allow the buyer to think its all original. To most people a slightly worn or even damaged original finish is preferable to any refinish, even one done by the factory.
If you're honest about it, by the time you pay for a refinish, the price will be higher than most people will pay, even for a Python.
Best option if you're selling is to sell it as-is and let the buyer decide whether to have it refinished.