If i get an unlocked iphone, will the stuff on my t-mobile simcard still be there?

I'm getting an iPhone off eBay and I just wanna make sure my T-Mobile simcard will work in it.

Yes! If you buy an unlocked iPhone, you can use it with T-mobile. But, unlocked iPhones are very expensive to buy. Better, you can buy carrier locked iPhone and get it unlocked by using Remote unlocking method. Many of the online vendors like http://www.prounlocking.com/apple-iphone/rs16wp9/ provides this servcie to unlock iPhones at affordable cost.

Yes, an unlocked phone would work with any GSM carrier.