You have direct communication with the seller. You can post coments, questions, etc in the ad. Essentially its the same as having it in a forum however the picture implementation is better as it auto resizes photos and makes thumbnails for you. Granted there are a lot of bells and whistles that probably arent needed and the auction stuff is completely optional. If someone was going to sell it on Ebay as an auction, they can list it there just as easily.
There is also a feedback system so if a seller ships promptly, etc, he can build good feedback. That helps the community as a whole because the good sellers/buyers can establish their reputation and it gives people peace of mind when dealing with them.
Overall I think it's an awesome package. We've actually had it for a while and I installed it once about 6 months ago and the previous owner of T6P looked at it about 2 minutes and said to remove it but with the forum integration working now (All new ads show up on New Posts), I think it's pretty complete.