You can find out who's online at the bottom of the homepage (Moderators appear in colour whilst normal members appear in white). Clicking on their names will take you to their profile where you can PM them, you can get contact details from their profile or by PM'ing them and requesting them.
First of all, take screenshots of any conversations you have with the user. Raw chat logs will not be any use - they have to be actual screenshots of the conversation.
Gather any other evidence you can think of/have gathered such as trade screenshots, PM's, threads etc. and follow the link Fendle provided above to make sure you have everything.
Post a thread here and tell the moderators as much as you can about what happened, and provide all proof.