wangyimeng, you have to make "real contact" with the actual person selling the knife, posting on a forum "I want it" or "I'll take it" without CONTACTING by direct means, the actual seller, is kind of a leaf in the wind.
Another interested buyer probably did send a direct e-mail (address available by clicking the original poster's name), probably got an actual reply from the seller. That's the way things work, "real actual communication".
I'm not involved in this transaction, but all of that may have happened even before you posted "I'd like it". You need to talk to the seller, not just post notes on a forum, or post your note AND establish direct dialog with the seller. Obviously more than one person wanted it, one made contact and worked out payment terms, it looks like.
You don't just post your address and wait for an answer, while others move forward to purchase, if you really want something. Best,
Frank H.