Your honor, my client confessed to the murder while in police custody, in fact -- we have it on tape. But we're not allowed to use it since it was not under oath. Shucks.
Short of using something like cash? I bet most transactions on Craigslist are done through cash (those under $1k or so). Correspond through CL or using relatively anonymous email, meet in public, use cash, and then go on your merry way.