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Because bitcoin.de expects a transfer to be made in 60 minutes.
How can this be?
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You are getting it wrong with the bitcoin.de 60 minutes.
It is 60 minutes you have as a buyer to confirm that you actually sent the money via your banks ebanking.
If you dont do that confirmation the trade gets canceled, you get a negative rating and the sell can sell the bitcoin to someone else.
The system is not really well explained on the bitcoin.de site as I have to experience on a daily basis with my buyers there...