I am pretty sure there is a way to get a temporary permit good for a few days/weeks that will allow you to export the car without ever paying the sales tax. I tried to search the DMV website for more detail but right now it's down -- sounds like business as usual at the DMV!

Edited to add: The site is back up. There isn't a lot of wisdom I can find about this but there is a chance that if the seller goes with you IN PERSON to the DMV you might be able to get away with it. Take a look at http://www.dmv.ca.gov/vr/export.htm