Can I buy an iphone from a different carrier and unlock it to use it on my carrier?

I'm currently with Tmobile (which doesn't have any iPhones yet) but the seller on ebay is selling his iPhone for Verizon carrier. Can I buy the iPhone from him and unlock it for my carrier?

No you have to buy it from t mobile but you could get a samsung galaxy s3 I got mine for christmas and it was only $50 from tmobile and now they came out with s4 the s3 went down in cost

If you want to use an iPhone with US network, like T-Mobile, you either need to buy an AT&T iPhone and get it unlocked to use with other networks or buy any other network iPhone from outside USA and get it unlocked. Verizon and Sprint USA phone even if unlocked can't be used with other networks within USA apart from the respective networks.