More Common Normal Map, OpenGL or DirectX

It's just a simple matter of inverting the green channel on the normal map and most paint programs can do this without a problem mate. If the end-user is buying models from various sources for their games etc then I'm fairly sure they will have come across this problem on many occasions and know how to right it. As Lemonade says above, just state OpenGL/ DX in the description. The worst-case scenario is they will PM you and request you to provide the one they require.

Both orientations are equally common, it depends on what program you use. You can offer both orientations, or just one of them, since convertion between them is trivial. Just make sure that you clearly indicate which normal map you're providing, since it's not always easy to tell that just by looking.