It seems there is no control over what is sold bearing the Ultrafire brand when it comes to 18650 batteries.
I am told that they can be bought in China for $0.80 a pair (yes around 25p each). It's no surprise to see them turning up really cheap on Ebay, but you should be aware of the possible dangers.
One unsuspecting individual who put their faith in the Ultrafire brand had a rude awakening when one caught fire:
Melting the exterior of the charger (Trustfire TR-001):
Under the wrappers you are likely to find:
recycled laptop batteries
Or an 18500 disguised as an 18650
Or even worse
If you are going to use Ultrafire batteries, keep a fire extinguisher handy.