What does bad imei mean?

I want to buy this iPhone for cheap, says it has bad imei for T-Mobile but can be pues off with a few dollars. What does that mean?

How do I get the video to show up in my camera roll?

Someone please help. So I downloaded a video from my PC into my Dropbox onto my iPhone and I saved the video to my camera roll but it has a circle with an explanation point, what does that mean and how can I get my video to show up?

How to stop a payment on a stolen/lost iphone?

How do i cancel a contract so i can stop paying for the iphone when i lost it. What do I do? I tracked my lost iphone and it says its at someone's house but i put it in lost mode. Do i have to call my carrier to report it lost, Will they give me a brand new phone for free, or can i just cancel off the payment because I don't want to pay for a phone that I lost, and I know the police aren't going to do anything.

I'm doing a project for my engineering class and i need

I need help figuring out one of the following
-Whats the market opportunity for an iphone (must be a specific one)
-a new or existing problem that apple is trying to solve with an iphone (specific one)
-or the challenge the solution is trying to fulfill
I only need to answer one of those and really need help.