I believe the OP was concerned about some fly-by-night outfit who makes a ten cent piece of replacement screen and markets it as "compatible with xxxx". The lack of quality and reduction in resolution/pixels could be expected there vs. an OEM screen where one expects the same quality as the original equipment had.
Yeah, that's a given. The point is, OEM can be used two different ways. One way is to say the parts are Apple branded. Toshiba might make the part following specifications supplied by Apple, and then the part gets a label that says Apple. In this case Apple is the OEM.
The other way is to say that the OEM is the company that makes the part for Apple. In this second case, the factory could make a run of parts for the 'aftermarket' and use the term "OEM" in their add copy and not be guilty of fraud.
However, just because a Chinese vendor says their parts are "OEM," does not make it so. In fact, I would think that only Foxconn and its suppliers could make that claim, and I seriously doubt Foxconn is dumping parts on eBay....
HINT: China is notorious for producing counterfeit goods.