with 50k+ in cash it probably is a bad idea.  Exposure to civil asset forfeiture/having to explain the source of the cash.

I would do a transaction such as this in my bankers office and the cash would be counted and deposited to my account immediately by the banker.  The casascius would then get handed to the buyer.  You still have the issue of a potential suspicious activity report from your bank and you will have to report the 70k as capital gains on your tax return.  For that reason, and the possibility you are accepting stolen funds, I would expect the premium on a transaction like this to be 30% or so.