Brian,
... Had he sent it with proper tracking & verified delivery via Post Office, UPS or FedEx, then you'd know what happened, & he'd be covered for delivering it (or she).
As of now, he has no proof that he sent it - if he did (or she) - so they need to get involved, or rollback the transaction (I'm pretty sure that will be State[s] laws &/or the federal UCC laws - but you'll need a USA lawyer for those if pressed to sue).
Start with you & the seller both contacting the Post Master at the post office where the seller is located, & also at the destination (Sandy UT IIRC) - to see if they can track down where it is - or if the seller then admits to not sending, losing or not having it in the first place.
If the seller has defrauded you, don't hesitate to report the crime to their local Police & their state's DMV - & tel;l them you'll do so.
If you got a Bill of Sale or something from the seller, then you
may be able to get a USA based DMV specialist to help you get title form the DMV in the seller's state to then transfer to Utah or the UK.
I don't know if Donna Leek does DMV work outside of Cal., but she's helped several classic car/truck & vintage trailer folks whom I know with CA-DMV problems, but it may be worth contacting her, since she may be able to refer you to someone in that state even if she doesn't do it herself.
http://dmvgal.blogspot.com/Cheers Mate & Good Luck!
Tom
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