Have you looked at the Sell Your Item form? It's right up there. Just click on Sell.
Then after you have reviewed the form, click on Sell again and choose Sell An Item for a webinar.
But before you try selling, build feedback by doing some buying. Usually we recommend mailing supplies like poly envelopes or multi-size boxes or the ever popular metric postal scale. This will give you an overview of how the site actually works.
