First, go to the forum you want to post in.
Next, find the button on the upper right that says "Post New Thread".
Look over several recent threads for hints on a good format. Start by giving your terms, such as how you would like to be paid, how you will pack and ship, and what rights you give the buyer for an inspection period and return.
Then list each item with a description of interesting characteristics and your asking price. To post pictures, I use
www.photobucket.com which I found easy to set up an account, and find the URL under each picture to copy and then paste into your sale thread.
Once you think you are ready to go, you can return to this thread with a sample ad and we can critique it for you. Once you ge the basic format down, each new sale just posts the specifics of the particular knife in the same way.