There is an problem shown in bitcoinwiki:
Someone runs both a money exchanger and a site meant to trap people. When Mr. Doe buys from the exchanger and uses those same coins to buy something from the trap site, the attacker can prove that these two transactions were made by the same person. The block chain would show:
Import coins from address A. Send 100 to B. Authorized by (signature).
Import coins from address B. Send 100 to C. Authorized by (signature).
Bitcoin transactions do not have a "from" address, but if the attacker believes that address B is controlled by Mr. Doe because the attacker received $5 from Mr. Doe's PayPal account and then sent 100 BTC to that address then they can infer the identity of the party sending to C. This assumption is not always correct because address B may have been an address held on behalf of Mr. Doe by a third party and the transaction to C may have been unrelated.
I can't understand several issues. Is "someone who runs both a money exchanger and a site meant to trap people" attacker? If so, than I (as attacker) already know the owner of address B, don't I? why I need trap site?