I'm the Digital Marketer at Flippa. Here are a few things you can do to promote your listing externally.
Do you belong to any domaining or webmaster forums that allow self-promotion links? Do you have business or personal contacts who may be interested? LinkedIn groups for your industry are also a great place to post.
One of the best things you can do for visibility is to apply the Featured Listing upgrade to your listing in the last 3 days of the auction. This puts your listing on the Flippa homepage, and typically attracts many potential buyers.
Depending on your site, you might also consider applying the Premium Listing upgrade, which gives your listing maximum visibility at the top of every search page. This option is more expensive, but it gives great results for websites with a higher reserve.