I've installed a mod to the Forum that offers a rudimentary auction function on the Swap Meet board. At the top of the board's message index, you'll see another button that reads "New SMFbuy item", to the right of the "New Topic" button. If you start a new thread using the SMFbuy button, you'll have the ability to set a starting bid, reserve price, shipping price and how many days the auction will run (up to 21 days.)
The other mod that I just installed allows you to add tags to your post in order to make it easier for people to find your topics in searches. You can add up to 10 tags for your post, separated by commas. I also re-installed the ratings feature, allowing members to rate threads.