Predi,
Fuel pressure regulators use the vacuum from the engine to control fuel flow. Most factory fuel pressure regulators are set more lean so that the ECU can measure the exhaust as lean and increase the duration of the injector pulse, this is better for fuel economy and emissions.
Having an adjustable regulator gives you the flexibility to have the engine run a little rich before the ECU corrects the injector pulse from running rich.
As for e-bay...
I would only buy from e-bay product that I had researched and was prepared to buy from a shop.
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