Self slabbing would protect the coin, but you may wish to have it professionally graded for "show and tell" sessions. I know I would. I think I'd give it a soak in acetone to see if that black spot could be removed before slabbing it.
a coin doesn't ever have extra value just because it is slabbed. you normally have to eat the cost of the process of sending it in and don't expect a buyer to make up those fees. For example, I will never pay an extra $50 for a coin to cover someone's fees who sent it in for grading.
Because so many people feel it is something "you just do" when you find an old coin in nice condition. I hardly ever submit raw errors to grading companies. I'd rather save the money and get more coins