I know I have asked this before but I am getting ready to purchase a new cell phone somewhere (ebay, etc) and just wanted to make sure I get all my info right before buying the wrong phone.

What exactly do I need to look for when purchasing one online. I know it had something to do with the frequency or something? Maybe CDMA or something?

Someone if you don't care please tell me what all to look for. I am probably gonna get some version of Blackberry....

Thanks in advance.