Although the electronic design of the Arduino boards is open source (Creative Commons CC-SA-BY License) the Arduino name, logo and the graphics design of its boards are a protected trademark of Arduino LLC.
In general the name and Arduino logo MUST NOT BE used to identify products or content, commercial or non commercial that are not released by Arduino LLC.
If you have a group, a blog or just an Arduino enthusiast you can use the Arduino Community Logo.
Officially approved boards are sold only by authorised distributors, you can see a list of them here http://arduino.cc/en/Main/Buy
Please note that Arduino doesn't sell directly on Ebay or Amazon so you should always check that what you're buying is authentic.
If a seller is not listed on our site, chances are that they boards they sell are not original.
For any questions regarding trademarks please write to trademark(at)arduino.cc
It is possible to sell boards derived from the Arduino design as long as these rules are followed
ie. If you sell a sensor that is compatible with Arduino you can call it "XYZ sensor for Arduino" and NOT "Arduino XYZ Sensor"
We have a dedicated team to manage the use of the Arduino Trademark and they can be reached at trademark(at)arduino.cc