Buying Cell Phone off EBAY

[esknights]

Hello all,

I am in dire need for a new phone, but I am still under contract with Cingular and can only upgrade for full cost. So I am looking at EBAY for some phones. Anyone have any useful info on this? I have the SIM card from my phone and from what I understand, as long as I buy an unlocked phone that is Cingular compatible I will be fine. Is that right?

Thanks