The only way you might get in trouble is if you get a CA DMV "ferry" registration. I'd just check with your state to see if you can obtain one and you should be okay - how would they know otherwise? The seller will file a sale form with CA DMV and they'll know the car was sold (so he's no longer on the hook for any tickets, etc.) but beyond that if you're out-of-state and they can't prove any use in CA, how can they go after you?
The absolute best thing you could do is buy the car and have it shipped.
AFAIK you could tell CA DMV and Franchise Tax Board to go F themselves with a rusty steak knife at that point as it's an interstate transaction and not subject to their taxes or jurisdiction and there was no "use" in CA regardless.