The difference is the quality and the grade. I can't say much about iFixit replacement parts as I have never used them, but I presume that they are A grade original replacement parts. When it comes to eBay and amazon you come across different grades and most of them are bootleg. We have sold hundreds of iPhone and Samsung screens to factories in China which in turn refurbish them and sell them again on the market. I hope this helps you if you have any more questions or want me to elaborate please feel free to ask. Regards
Mechanical snap could be related to your hard drive being faulty. Try disconnecting your hard drive if you have all the required tools to complete this procedure. You will need suction cups and the correct screw drivers. Update: The mechanical snap you wrote about might be related to your hard disk, try disconnecting the hard disk and try to power on the mac again. Regards