Thank you for your advise, Callum and S. I managed to file a Dispute Report, thank you.
I bought an item on the 7th of August. I emailed the seller asking for payment instructions and asking him to pack well and send me the tracking number after he posted it. He replied promptly and I paid straightaway. After 2 days, he emailed me saying he couldn't see the money in his account, therefore I rang my bank, I emailed him back that the money had definitely left my bank and asked him to check his bank. After that email, he never replies to my emails anymore. I suspected it's a scam and tried to contact Trade me, I don't know why but a message "No eligible or recent purchases"...