I doubt that you can sell such a 2D shooter as PC standalone game. There are too many better alternatives.

But there are other platforms that have a market for that:

-Java Mobile phones
-iPhone
-other small handheld gaming consoles

They can also work as Flash Browser games.

Some small recent games of mine:
Griefer 4k http://damocles11.byethost9.com/griefer4k.html
Kobold 4k http://damocles11.byethost9.com/kobold.html
Cave 4k http://damocles11.byethost9.com/cave4k.html