I've also used the Birchwood Casey Aluma Black, it works great. A sharpie will fade and/or dissolve with cleaners and wash away.
From what little research I've done, there is no way to "SPOT" re-anodize an area where the anodizing was damaged. You'll just make a mess of the rest of the anodizing coat trying to do it. There is spot anodizing equipment, but its only for aircraft landing gear, cost and expertise looks like its impractical, besides it repairs the coat to prevent corrosion, probably makes the repaired area look worse than it was before.
Besides, the TypeIII hardcoat that is on the receiver now is a process you can't do at home, it takes really expensive hi-tech equipment under a very precisely controlled process, you can only do TypeI/II and that is much weaker than Type III, plus it make a mess of the rest of the coat.
All you can do is cosmetically fix the dings and scratches, the Aluma Black does a good job doing that. It will help prevent corrosion, but it won't offer the hard layer of protection the typeIII anodizing does.