There are a lot of sites out there claiming to have OEM parts out there. The words OEM are very loosely used out there these days. I have found that most of the parts on this website are truly OEM or as close to it as you can get: OEMcellphoneparts dot com
Most parts in the other sites you referred to are used taken out of other defective phones or cheap copies that may look and work well initially but rate of defect is much higher.
Hope this helps.