Isn't there some technique whereby you follow the real world around and marry it up to the transactions in the blockchain?

i.e.

I buy a thing for £123.45 on the 22nd of October at 12.34pm {-- law enforcement can prove this, but can't prove the seller

There's a transaction for £123.45 on the 22nd of October at 12.34pm {-- law enforcement can also prove this, but can't prove that I'm the buyer

One transaction ain't gonna cut it, natch, but if there's a big old pile of transactions that line up, then that's starting to look like the kind of evidence that would convince a jury.

Or something. I'm bound to have all the details wrong here.