How much of this is due to anti-money laundering regulations enacted in the early 2020s [0], which much better vet the identities of the buyers/sellers?

[0] https://www.lseg.com/en/risk-intelligence/financial-crime-ri...

That reminds me that I looked up gold custom iPhones once.

They had various payment methods, including cash (highest price on their list was 300k usd) at their office in Dubai.