Shutterstock, Dreamstime, and iStock allow you to sell your vector illustrations on their site in the same manner as the digital photography. This means you can create imagery in Adobe Illustrator, Freehand, Xara Xtreme, or Inkscape (free download). You earn money for every download from their subscriber base, with additional incentives for referring buyers or sellers to the site.

This sample image is one of my most popular vector images on Shutterstock, from a series on road signs I made. I drew it in Illustrator and saved it as an EPS.