In Medford, just across the border, there are 2 dealers. Naumes is the one that sold Ultrarunner his
BX23 for $14,900. No sales tax in OR is correct (but they don't let you pump your own gas up there-go figure).
As far as Board of Equalization goes, cars you already own in another state, when brought to CA, are charged vehicle registration AND 7.5% of their value extra, just as if you'd just bought it here & had to pay sales tax a second time. Now they can track that 'cause you have to register the thing. Insurance agents have told me that bigger tractors here have to be registered, but that BXs are too lightweight. They're considered personal property, so no registration. Technically, for anything a Californian buys mail or internet order from another state, you're SUPPOSED TO fill out a form on it & pay whatever the sales tax would have been if you bought it here, and attach it to your state income tax return. Guess how many people actually do that? I've never heard of anyone taken to task for not doing it.
At the CA-Oregon border on I-5, there's an inspection station, but they can only ask whether you're bringin' in fresh fruits & veggies. They can confiscate that 'cause exotic ag pests cost us billions to eradicate. I don't think they can leagally ask about other stuff. That road goes over a BIG mountain, so you'd need an extra heavy duty truck AND trailer with VERY GOOD BRAKES. To avoid an inspection station, go from OR to NV, south to Reno, west to Truckee, CA. Then, instead of I-80 over Donner Summit, when you get to the town of Truckee, take the road to Donner Lake, drive along the lake & keep going. This is the old highway over Donner Pass. (Yes, for you curious ones, all named for the Donner Party cannibals.) There's no inspection station on this road, & you can get into CA without being inspected. Still, you better have really good truck, trailer, & brakes. You might think about renting a U-haul truck & puttin' the whole rig inside it, really secured well.
Good Luck whatever you decide. Let us know what you wind up doing.