When buying, selling or trading with another member of the forum, it's wise to take a look at their Trade Count rating and feedback.
To do this, click their username anywhere it appears on the forum (typically on the left side of a post they've made). Look for the "SMF Trader System" label and read the stats under it.
When you make a purchase from a member, be sure to submit a rating and feedback for the transaction. You can do this from their profile page as well, just click the "Submit feedback for (username)" link beneath the "Total Positive Feedback" label on their profile.