If you have concerns about a user, please make a thread in our Contact the Staff board. However, keep in mind that it's not realistic for the site staff to intervene when it comes to disputes about any in-game transactions, which is why we always recommend leaving a feedback rating for anyone you trade with.
This isn't to say that we wouldn't be more than willing to look over the situation and offer advice, so please feel free to make a thread detailing the situation in the board I linked up above and one of us will get back to you shortly.