Randall,
This thread may have some useful information for you,
http://www.renntech.org/forums/index.php?showtopic=6463, although they are talking about these 9" wide wheels on the rear, rather than the 10":
http://www.wheelenhancement.com/index.php?t=WheelDetail&id=43
If I understand correctly that the ebay listing says the wheels are built to the same specs as the wheels linked above, you might want to call or e-mail WheelEnhancement and ask whether these 10" wheels fit on the rear of a Boxster, and how they would do it if you ordered the 10" wide 996 five-spoke wheels from them.
Edit: I notice that the ebay listing has a "fitment chart" that states the offsets. Compare those with the offset info in the first thread above.