FLook under Forums/ the trading post There are instructions on how to use it, the procedure is meant to provide private conversations re. Transactions.
Can you explain exactly your problem? Often people think there is something wrong when they can't reply to an existing post in there, but that's the way that section is set up. You can only post an ad in there. After that all conversations are made in private, just as Ivan says. Not via direct replies as in the usual foras. But if you, as a subscriber, can't actually make a new topic in there, there is something wrong. @Spiritburner