My rules are simple.  I do not shoot:

Humans (difficult to get release from unknown people) and besides that I still do not know how to handle the editing of human skin in image.
Art scene like Vendors, something happening on the road. Editorials are no fun for stock photographer.
Conceptual images: I am not there yet. I did some but came back to my own ideas of not doing them until I get the people to be in concept.
Tree, flowers, vegetables spikes, cactus etc. I learnt it hard-way when my entire shoot of flower-show was rejected completely.  So, it is another no-go area.

I guess you shoot in jpg. If your camera can do raw and you can get hold of a free raw converter, the simplest way to edit skin tone is by adjusting the color temperature. I do two steps, first I set the temperature to neutral (by pointing the temperature picking tool to a white area in the photo) and then I make the image a little warmer (higher temperature). That usually makes the skin nices.

You can also look at the number values of skin tone. For the average caucasian skin ie, you hould usually have the numbers of yellow and magenta  close together with magenta a little higher.

On the other hand my old photography mentor would have said: if you have your lights right, then no need for editing.

do you work with studio lights? Or with available?

Fotolia brings the most money to me considering the number of images accepted. Shutterstock as usual the highest at all.

You just have to get used to random rejections. I had rejected this week for 'poor composition':

And just got the reply from Scout telling me that "Ducks is a category already well represented in the iStockphoto collection.Therefore, we are now only accepting the most exceptional examples of the genre and those that are of the highest technical quality. We do not feel that this file successfully achieves this requirement."
The title, description and keywords say nothing about ducks. It is a Shearwater, not even close. Grrrr. There are only 10 files tagged 'shearwater' in the whole of iS, and two of them are Puffins. None of my species.
I know it wouldn't have sold much, but the file is there, it's already cost inspection money and Scout money, (and it isn't a duck).
 
I'll get over it! I accept it's not the world's best bird photo, but shearwaters at sea are difficult, as you can't get near to them. But if someone wants a photo of a Pink-footed Shearwater, they won't be fobbed off with a photo of a duck, no matter how beautiful.

Added: that photo was subsquently overturned by Good Scout.  But given the note about not wanting any  more duck pics, many ordinary mallard shots were still being accepted even before they lowered acceptance standards.