Here's information specific to the classified section: Marketplace Posting Rules
Once you're ready to go, click the "Classified" tab at the top of the screen and choose the "For Sale" section, then "Forum Tools" and "Post a New thread." It's good to include year, make, length, and price in the title
. Make sure to choose a prefix for the state. Pictures draw more traffic and interest.
As a courtesy, when the trailer is sold, add a post at the end that says "SOLD" so the ad can be closed and moved to the archives.