Basically, people on this thread are saying "Umg, prity pikcha luk so gud, naise werk mattU". I really don't see the need for something like this.
1) You asked why it would make trading harder, it wouldn't make it harder just more complex. When people open a thread, they have the title, pictures of account, the trust level of the person they're trading, and the offers that have been made, all in the one window. In your current system, there's no contact details, you have to click to see the user, you have to click to see the discussion.
2) The thread system is what makes sythe unique from other runescape selling/trading sites. If we made an integrated system, I'd see no difference between this site and say, Usfine or Gold4rs.
Hope my feedback helped a little.