all sounds great Windy if you want to save a few pennies but tbh i wont buy anything off ebay that is of a life safety nature.
the recaro and vac stuff is tested and certified, not sure about your ebay stuff
if you look at Thorneys car when it hit the wall. his seat gave way on the side mounts so imagine the forces that go through these on impact
Simpsons used to make their own base mounts but thye were heavier an di think they just thought a proprietry product was more reliable