Posting a classified ad is pretty much like posting in a regular thread, which you have already done here. The only difference is the drop-down location tag for the state or province ("Prefix") in front of the title
. You might try going directly to the "For Sale" section of the Marketplace and use the "Forum Tools" menu to start a new post.
Before you do that, it would be a good idea to review the rules and recommendations for how to create and manage your ad here:
If you're posting from a mobile device using the FGRV app, things work differently.
Best wishes! Feel free to ask more questions if it still doesn't work for you.