Well, on 15 August 2012 I made a purchase of 10 BTC, I transferred 98 EUR to the seller, but then the transaction was suddenly having the status "cancelled"
That doesn't just happen. The seller will have to press a cancel button, which causes an email to be sent to you. Then, IIRC, there is a minimum of 3 days wait until the cancellation is accepted.
At the very least, you must have ignored the cancel message you got - that, or your mail provider failed to deliver it.