Buying iPhone 3G on eBay for use with T-Mobile?

[Broker]

I am looking to buy an iPhone 3G on ebay for use on my current network (T-Mobile) I know I wont get 3G speeds and just Edge, but I am on a wi-fi network 90% of the time anyway. My main question is do i have to buy an already unlocked iPhone on ebay or can I buy a locked AT&T one and jailbreak, then unlock it with Blackrain or Cydia? I dont have an AT&T SIM card, Is an ATT SIM required for unlocking it? Thanks in advance, and any unlocking hints would be appreciated

Basically, do i have to have a ATT SIM in the phone first, unlock it, then put my tmobile SIM in?

[Unregistered]

You can definitely buy an already unlocked iPhone 3GS and use it with your Tmobile SIM. But ensure that the iPhone was unlocked with a safe method. Certain unsafe unlocking methods would give you OS problems and wont work with Tmobile SIM. So it is better to get a network locked iPhone for low cost and unlock it yourself at low cost. You can unlock an iPhone using remote unlocking service. You can get the remote unlocking service for an iPhone 3GS from ClassicUnlocking.Com Good luck.