This is a good point. I think the best way would to be have a "preferred contact method" next to the username, which could be a link to that user's thread if they have one, or their profile if they don't operate out of a thread. Of course, this isn't for everyone, but it does allow those who want it a great way to showcase their wares in a highly visible location. :)the problem I see here is that people maintain (sometimes with great effort) trade threads here for a reason.
I know personally that I prefer people to contact me on my thread if they want anything I have. It allows for transparency that does protect people from getting scammed.
It's nice to put them all in an organized list but, quite frankly, I don't want people pm'ing me asking for trades...thats why i have a thread. So if you're really interested in doing this, please consider me opting out.