The ebay Wheel Enhancement wheels have the :
"Sized (2)18x8et 50mm and (2)18x10et 47mm"
You are looking at 911C2 factory wheels, which
may be different offset OR may be the correct offset.
...need to check with your seller about the offset and
look in your Boxster manual for the correct offset for
18" wheel sizes.
Good luck,
Ed
PS: most all can be made to fit with spacers