I'm Lucy, and I'd be happy to answer your questions today.
You have the ability to bring an action in ejectment against your neighbor, asking a judge to order them to remove the garage from your property. If that means demolishing the garage, they could be required to do that. Or they could be required to buy that strip of the land from you (based on value per square foot times number of square feet encroached). At the same time, you can go to the county property recorder's office and file what's called a "lis pendens," letting anyone who is thinking about buying the property know that they'd also be buying a lawsuit. This site has a template that you can use.
The lawsuit is against the owner of the property, which can change - you'll still be able to sue if the neighbor completes the sale, by amending the Complaint to use the new owner's name. But if they haven't done a title search yet, you may be able to stop the sale until your case is resolved (which actually also benefits the buyer). It would help to have a local attorney draft the Complaint for you, since this is something you'll want done quickly. The California Bar has a referral service that might help:
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