Bought Bought an iphone4 used and the previous owner did not had a protective film on the home button. Resulted in scuffed even a little chipped around the edge home button.
Had a mishap when dissassembling the wifi antenna, the bottom center screw was torqued too tight by manufacture and the base piece where the screw goes into had broken off from the motherboard. Luckily it does not affect operations.
Be patient and be very careful ripping the battery out from strong adhesive backings. Be patient separating the front screen from the frame adhesive.
I would suggest performing this task in the hottest environment for you and the phone so things will be soft and pryable. Instead of cold and brittle where things break.