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I wouldn't be concerned about the fact that it has a T-Mobile SIM card in it. What I would be concerned about is whether it is unlocked. I would try to use a local SIM card, and see if it works. If it does not, you might want to try to return the phone to the seller for a refund. Was the phone sold as being unlocked? Many phones sold on the international market are sold as unlocked, when in fact they are not.
I think it's a little odd you found a SIM card in their already. It's possible it may have been used by another person and they could have left it in there by accident. I agree that making sure the phone is unlocked is a priority so you can actually use it. It's a good idea to take the info in the previous post into consideration.