Dear Camilla,
This is an eBay issue, not an issue with your address. So you can use your California address.
Due to new USA Tariffs, items must have Country of Origin filled in. So you could contact the Seller directly and ask them to do this.
There are also many sellers who use the Global Shipping Program who have this glitch. Again please contact the Seller for help. Some people had better luck buying from ebay.com instead of ebay.co.uk
Other people were able to buy after the Item was ended and relisted. Also your Seller may be willing to offer a different shipping option other than GSP.
Good luck with your buying!