First, there are 3 kinds of Arduinos:
- The "official" ones actually made by the Arduino people or specifically licensed by them.
- Clones... These are not made by the official Arduino people and should not attempt to trick you into thinking they are official
- Counterfeit... These are cheap Arduinos that are NOT official but are being made to trick you into thinking they are official. Arduino.cc even has a link to help you spot these: http://arduino.cc/en/Products/Counterfeit
I was sold 10 of these by a marketplace vendor on Amazon. These were poorly made. The thru-hole parts (DC connector, crystal oscillator) were very poorly placed. The solder joints were crusty and the solder was very poor quality. The silkscreen was poor and not printed uniformly. Everything about these felt cheap and it was an attempt to rip me off. They came in brown boxes and included a USB cable. (Official ones DO NOT come with a USB cable.) I promptly returned them and reported the vendor.
If you are looking for an "official" Arduino UNO R3, then it should only come in a thin pre-printed card-stock box and should include a little booklet and sheet of stickers. If the box is bigger and comes with anything else, then it's not an official one. I don't mind clones or 3rd-party Arduinos, as long as they aren't attempting to trick you into thinking they are official. I've bought stuff from Sainsmart before and think they are generally okay, although I haven't bought their clones.