While I do like this image there are a few things that bothers me about it.

To begin we have the anatomy, it come off as very stiff and pose-dollish. I suggest you simply practice a more fluent way to draw your lines and that way get more fluent and animated feel to your anatomy. Because right now it comes off a bit... stiff as said. It's almost as if she is feeling awkward because someone is watching her. Another good way to get more alive anatomy is simply to pose infront of a mirror or to have friends/familly pose for you.

I like your choice of lightsource, though the color could need a bit of a better blend in the skin. The colouring on the dress works better actually then on the skin.

It could be that I am used to markers and that watercolour don't blend the same way, if so, please excuse my remarks on the blending.

As said by a previous poster, the hair don't have much shading and thus it just floats together and becomes the same colour. But I like the lineart there, it's not as stiff as the lines for the anatomy in general. Try to use the same flowy lines when drawing the body as well.

One thing that bothers me about the dress is the bodice. This is just a design question... While i know the Sailor Moon costume for Princess Serenity looks pretty much like this, it does come off as very flat. There is no depth to the designs on the dress except on a few areas like the pearls. There rest of it sort of looks like it's her skin rather then a dress. Best thing would be to have a millimeter or so stand out to say that the fabric is fabric and not just a tattoo on her chest.

The train and skirt is perfect though, I am impressed to be honest. Very lovely shading in the colour and in the fall of the fabric. Good work there.

I do hope my probably less then kind sounding comment helps a little.

Lovely piece, Good luck selling it ^^

So much detail in the dress! Oh my gosh.
I can just imagine how much effort and time you had to have put into this piece and it really does pay off.

I love the way you've combined realism with the stylized manga form. It brings a softness to the image that I think would otherwise be lost on the manga style.

The colors are well balanced with each other.

My only piece of criticism I guess would be that the upper torso on the foreground figure seems a bit too angular to me. It could just be that the left shoulder is a bit too pointed, thus creating a rectangular feel.

It's a very lovely piece. Good luck selling it!
Keep up the good work!