I just restored to 2.2.1/jailbroke/unlocked my iPhone.

I tried to use a T-Mobile sim card to test it.
The Emergency screen showed up but the upper right corner had a full signal and "T-Mobile" was in the carrier spot. It also said "Different Sim Card Inserted" and iTunes would not recognize the phone.

Now my question is if i try to sell the phone on eBay will ANY AT&T sim card work in the phone?
OR
How can i get the T-Mobile sim to work? Because i checked the Unlock box when i jailbroke it with QuickPWN. The BootNeuter was successful and everything. I dont see y the T-Mobile sim didnt work.