Can i buy an unlocked/jailbroken iphone and use my T-mobile sim card in it?

Yes. You can definitely buy an unlocked iPhone and use it with your SIM card. You can get some good deals on unlocked iPhone models from ebay or amazon. Before buying an unlocked iPhone make sure from the seller, how was the iPhone unlocked. Certain unsafe unlocking methods would give iOS problem while working with other SIM cards. Temporary unlocking methods will also relock the iPhone on your future iOS updates. Resulting in more expense. So it is better to buy a network locked iPhone for low cost and unlock it safer and permanently by yourself. You can unlock an iPhone using remote unlocking service. You can get the remote unlocking service for an iPhone from SuperUnlockCodes.com This method of unlocking is safe, simple, affordable, quick and permanent. It doesn't require a jailbreak too. Good luck.