What is an OEM SSD for Macbook Pro
OEM means Original Equipment Manufacturer.

Manufacturers (like Apple, Microsoft, Ford, etc.) don't actually make every component in the devices they make.

For example, Apple doesn't actually make the memory, the screen or the SSD that's inside your MacBook.   They are made by other companies like Samsung, Micron, and LG.

When you purchase an OEM product (i.e. a drive), you're purchasing the same component that would have been installed.

What you are seeing on eBay are usually "pulls" from salvaged machines.  The drive they pull from a salvaged MacBook Pro, for example, will be listed as the OEM drive.  

They are usually more expensive because there are very few 3rd party options avilable (i.e. Samsung doesen't offer PCIe SATA drives with the Apple spec or LG doesn't offer "Retina" quality displays elsewhere; usually all by contract)