Does bitcoin provide non-repudiation?
If you used bitcoin to buy "guns", how can the court legally prove that you, a person, are the owner of the key that made the transaction?


The Ross Ulbricht/DPR trial provides several examples of how you might prove that someone made a transaction:


Being the person who sells the product or service to them. For example, the "hitman" Ross hired to kill Curtis Green was actually an undercover agent.
Grabbing the laptop used to make the transaction out of the hands of the guy who made it, and forensically analyzing it.
Other ways, like having a confidential informant, (like Curtis Green) or getting them to plead guilty (like Carl Mark Force.)