Various things come to mind here, quite apart from what you were asking about:
* unitedairlines.com.au is sometimes cheaper
* it's often cheaper to book Australia-SFO and SFO to anywhere else on separate tickets
* you can get the taxes removed from the domestic ticket attached to the international
(downside: that means booking it direct with UA once the main int'l is booked; upside: a good time to use up bump vouchers, etc)

If you can't select the seats you want online, it's not a big deal. Phone UA and get 'em changed afterwards.