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The KYC and AML measures don't prevent you from paying with cash for things, even in large sums, but they require that parties involved - sales agents, banks, solicitors etc - verify that you can convincingly document that the cash is a legitimate income.
Also, if you do buy expensive things for cash - such as cars and houses - the seller will be at risk of being paid with counterfeit bank notes. That's another reason to avoid cash.