Per post #34 .. is it possible to hook fuel pressure gauge directly to pump.. bypass the filter.. and confirm a low pressure there? If you can and still shows low pressure contact your ebay vendor.. hopefully some kind of warranty if the pump is not pressurizing enough.

WARNING: THE FUEL SYSTEM IS UNDER A CONSTANT PRESSURE
(EVEN WITH THE ENGINE OFF). BEFORE TESTING OR SERVICING
ANY FUEL SYSTEM HOSE, FITTING OR LINE, THE FUEL SYSTEM
PRESSURE MUST BE RELEASED.
With the ignition off.
Install a fuel pressure test gauge to the fuel rail.
Start the engine and observe the fuel pressure reading.
Note: Fuel pressure specification is 3.7 to 4.2 bar (54 to 61 psi).
Choose a conclusion that best matches your fuel pressure reading.
Within Specification
Go To 6
Below Specification
Go To 14
Above Specification

Replace the fuel filter/pressure regulator.