The shipping thing is a good point. I did that with our current one. I found it for sale locally in rural Maine, about as far from San Francisco as one can get and still be in the lower 48.
A phone call and photo of my ID later we agreed that they would box and ship it. Fortunately it was coupled came with the box+wheel bag, and the owners had lots of experience traveling with it. They gave me dimensions and weights and I purchased and emailed them the shipping tags. FedEx (BikeFlights or ShipBikes) picked it up from them and for under $200 it was delivered, insured for full value, to my house. The backup plan was to fly out to get it, but this was easier. I wouldn’t buy a bike unseen for a first tandem, but I would definitely consider flying out to see one and bringing it home on the plane.