There are a lot of different icon sets on the iStock (and around the web). Why do some of them have perfect sales and some of them barely have few? If you're keen on icon design as I am and want your icon sets to be successful, please remember just a few rules:

Don't be lazy. do a sketch
Choose right colors. Believe me, it is better to spend one day for choosing them than spend a day/two/a week for drawing icons which nobody will need/like...
Use the same style for all icons in the icon set. That's not right to use 2d style for 10 icons and 3d style for another 5.
Choose the icons' size before you start drawing them (for example, 48*48px or 64*64px). Keep using this size for all icons in the set.
If you are drawing a small icons (16*16, 48*48, 64*64 or around that) it is better to use whole numbers for dimensions of those parts that you want to be distinctly visible (edges, for example).
Use whole numbers for X and Y coordinates. Check it if you notice that an edge looks blured. Because even if X=5.001px it will look blured as much as for X=5.5px

Use "pixel preview" in the Adobe Illustrator. It's very usefull to create two windows for one document. Just go to Window-New Window. It will create another window for the current document. Use Alt+Ctrl+Y (View-Pixel Preview) for this window. If you don't want this additional window, you may just switch preview mode in your main window.

If you want to sell your icons successfully, do not scale final icon image before showing it to the customer. If it has clear edges in 48*48 px size, it may not have them as soon as you scale it up to 64*64 or more.
Pay attention to details but don't go too deep.