I own a general website directory which is google page rank 3 and receives a few hundred visits a day. It offers free link submissions of which there are around 25-100 per day.

It also offers featured link submissions for a one off cost of $5. It makes one of these sales every few days.

What free ways can I promote my site, especially towards an audience looking to purchase featured link submissions from me?
My site is www.hsmarket.com

promote your site though other mediums. Social Media is very hot right now and it is all the buzz. Try making a blog and get links to blogs on similar topics. Open a twitter account to update clients about any deals you are having for link submissions that month for example. Also why not put your website address on here. If you left a link I know I would have clicked it