Very nice Dustin, finally a classifieds site that listens to suggestions. And by coincidence, I'm full of them...
I think classifieds sites need to monitor ads being posted, for fraud or shotty sellers. Fauna has become a necessary tool for people who might be buying from someone on KS, and need to know some background on this person. But this is only because KS fails to do so itself, and would rather profit then make sure it's users aren't ripped off. Perhaps a ratings system, like ebay has. And a complaint forum on the site where people can openly report their dealings with bad sellers. This would make your site a reliable place for people to buy reptiles off of, a site with some integrity for a change.
Most forums also have moderators to keep the boards they moderate clean. Perhaps a classifieds moderator would be a good idea. So if any seller with an awful reputation for ripping people off posts, they can be taken care of.
Besides that, get an easy to navigate design that is easy on the eyes. I tend not to surf around a site for too long if I think it is overly cluttered, or just plain ugly.
Hope this helps, and best of luck.