You could try to sell it the way it is, since flat black is currently popular.

The drawback might be that older buyers want a "nicer" car, and flat black is a low-buck alternative.

The next question is, can you paint it yourself? That will cost $1,000-1,500 for good materials. If you have to have it painted, you will probably spend $5,000 and up, unless you go to Earl Schieb. Can you get that much more for it? Otherwise there is no point in painting.