Tracking pixels are pretty standard for the web analytics industry, and are a tool to aggregate information, not sell. Google, Facebook, Optimizely, Marketo, etc. all use them.
If you really want to learn more, you can read the wikipedia page on it: http://en.wikipedia.org/wiki/tracking_pixel
You can easily validate this by checking all the "pixels" (in the RTB sense) that are being deployed by Google. In their defense, they do mask a lot of things so stuff like the full IP address are not available (last time I worked on that they just give you the three first segments) but still quite a few information was available to target your demographic segment.
Edit:
For downvoters and people curious about tracking pixels: - https://support.google.com/dfp_premium/answer/1347585?hl=en - https://support.google.com/dfp_premium/answer/2508388?hl=en