Apple iPhone 3G - 16GB - Black Repair ?

Here I am selling iPhone 3G 16 GB There were few scratches on the back of the screen. I was trying to be smart. Bought a new back case and tried to replace on my own using the Guru (Google), Everything went on fine when I assemble it but it is not working now. There was a slight gap at the top of the panel as I got fed uo doing it The back panel is brandnew but the front one old, so there are few scraches. I am selling Phone only. It is ideal for people who knows what they are doing. I am selling this as a repair/Spare, so there wont be any returns. Please give me a shout if you need any info before that.