Problems activating new iPhone 4S

[danetello]

I decided to buy my iPhone 4S on ebay and I have received it today. The phone was brand new and sealed, when I tried to insert my UK 3 sim card to activate it, it shows this message:
Only compatible SIM cards from a supported carrier may be used to activate iPhone. Please insert SIM card that came with your iPhone or visit a supported carrier's store to receive a replacement SIM card.
The seller said it was unlocked, does this mean it is locked to a network?
Or will I need the seller to activate it with his sim card and then I can use any network?

 

[Obscurelight]

You were sold a locked phone. Return it to get a refund. By the way where is the origin of the seller?

 

[Saileshbreaker]

Yes it is locked to a carrier! From which country did you buy it? If USA then its most likely on AT&T

 

[danetello]

Ok thanks for the replies, is there any way I can unlock it myself or at least activate it? If I just bought a micro sim from the shop and tried to activate it, would it work?
Also it was bought in the UK from a UK seller.