It might not be any harder than doing it without Bitcoin. Instead of issuing a cheque in the name of the notary (potential technical troubles, high fees, etc.) or making a wire transfer (assuming notaries work that way in Germany), you will mail in the required secrets. Assuming there is a foolproof way of making it work, that is. 
That is exactly where a BTC escrow comes in. If you write a cheque you know the money is with you, and the buyer knows they have something to get to the money.
With BTC I can give you my private key but there's no knowing when you move the coins using that key and there is nothing I can do about it. Also, you might close the deal and then I move the coins myself (we both have the private key) and there's nothing you can do about it. With escrow none of us holds the private key, and we both have some recourse if something goes wrong (granted, it depends on how much one can trust the escrow). In the end, escrow service is the cheque in normal currency.