Working as a freelancer or not, every designer is able to make some money from actually doing nothing by putting his work for sale online. Of course a dedicated website would pay 100% of the price of whatever you wish to sell but that would require a good pagerank, search engine visibility and many clicks, to make a satisfying income. Thanks to dedicated stock sellers though, there are already tons of places with many visits, so it all goes down to creating elements that people would actually buy.

Some places to upload and start selling your work

iStockPhoto
On iStockPhoto you can upload practically anything. Photography, icons, backgrounds, vectors, video and recently audio. It already has thousands of customers and is the number one choice for anyone who wants to find something to enrich his project with.

DeviantArt
It is an art community but you can sell prints of your illustrations.

VectorStock
Anything vector. Icons, illustrations, backgrounds, textures, web templates.

Ultrashock
Flash, images, vectors, audio.

FlashDen
Anything flash. Videos, websites, various elements. Pretty good for flash lovers.

Crestock
Photography. Great for creating ads.

Revostock
Video, audio, photography.

ThemeForest
Web templates, WordPress themes. Good commissions. Great for portfolio templates!

WP Theme Market
WordPress templates

Premium News Theme
WordPress templates

Lulu
Books and printables.

Tips

• Most web designers usually buy three things: Icons, backgrounds (preferably seamless) and photos for ads. A good piece of advice would be, take a look in the above websites, see what sells the most, figure out what’s missing and create work based on that research. There is always a need for grunge or “hand drawn” icons. Floral patterns will never be too retro to be used. And nice clean photos of smiling people using computers is a great basis for IT related ads.
• Create your designs in vector format. This way, you can practically sell them anywhere, by transforming the vectors to flat images or PSDs.
• Read the terms of agreement! Some websites do not allow their uploaded works to be featured elsewhere! Develop a strategy and if you’re gonna use them, upload only what would sell the most on that website.
• Designing web templates and turning them into WordPress themes is always a good idea. Again, read the terms. Sites that sell themes don’t usually like sharing their crown jewels with antagonists.

That would be all for now. Feel free to send websites that you use so the list grows, and we will update it. Just remember that some people make huge amounts of cash per month by only selling to sites like these. Find the pulse of the market and use it to your advantage.