Lesson #1 is, don't buy some small items from one seller then switch your ebay shipping address to the USA reshipper for one more large, heavy item from the same seller, then try to change the earlier item's shipping address to the same re-shipper to save some money due to combined shipping.
It messes up ebay's anal-retentive micro-managed invoicing system rather spectacularly.
That one was my fault, and I've certainly learned not to do that again. Hopefully it's sorted out now.
It turns out you (the buyer) can alter the ship-to address of an item after buying it but before paying. It's down the bottom of ebay's 'pay for item' page, though badly worded.
The next one, is some sellers require a US phone number on the ship-to address, even for USPS parcel post. So despite that you're asking them to ship to practically next suburb in California, there has to be a phone number - ebay *insists*, by not letting you pay the invoice till there is.
I was considering paying some money to a service like
http://www.tollfreeforwarding.com/california-phone-numbers.html (gives you a real phone number in the State you want.)
But their terms of service were kinda terrifying.
Next idea was just to use one of the phone numbers they list as available for rent, and so what it if doesn't work. Not like USPS is going to be calling it anyway.
In the end the seller suggested just entering 888-888-8888, and lo and behold, ebay accepted it.
So far (with two sellers) I haven't noticed any tendency for them to think I'm a scammer. Their listings both said 'ships world wide'. The main problem has been they are both high volume sellers, and that makes their attention span gnat-like, with little motivation to deal with 'problems'.
Next experiment (if these two work) will be to try a simple purchase with no 'changing address' dramas.
Then after that, a purchase from some seller that specifically says 'ships to USA only', but isn't selling anything with legal limitations on selling overseas.
It will be very satisfying to be able to get around those guys, and whatever their problems are - xenophobia, can't be bothered to prepare customs forms, etc.