Motivate facebook fans to use actual website?

Maybe give them an incentive to do so?

Run a contest? Have a free giveaway?

I assume it's free classifieds but if it was paid you could offer a discount on your fees for a limited time.

Post updates 2-3 times daily to "interesting" listings on your site.

You could also make some videos showing them the advantage of posting onto your site.

Why should they? How many visitors do you get to the site? Is it worth their time to post an ad or is the place pretty much a ghost town?

If your site looks unused, it's going to be difficult to get people to post listings on there because no one wants to be "the first one".

If that's the case, hire someone to post ficticious ads to make the site look active and "filled up". Even mark some items as sold if you can.

I did this with forums a while back and it worked.

Also, if you haven't already...create a landing page so that they become a fan you instruct them that the "next step" is to check out your site...and provide a link.

Tell them "it's easy to post an ad". Talk about your visitors and give it a community feel. Again...people don't want to be part of a dead site. Not saying yours is...I'm just assuming based on the info you posted.

Hope this helps a little,
-Peter

Create a few applications and discount coupons and integrate them on ur fan page as an application or via I-FRAME..

Make sure that ur website looks good and hold weekly contests on ur facebook page.

engage them with MP3 players, opt-ins .. Provide them some free e-books or a few tips related to ur niche!!