[kilian]

First off, your user is not you. ;) Something that's instantaneously understandable, and clearly better to you (tagging) isn't necessarily so for others (apart from "multiple categories per item", I still don't get why everyone loves tags).

Obviously you can get quality feedback. Iterate on that. If it's not clear what problem you are solving, you just tell them. There's no shame in writing "This is a listings page, add a listing HERE" in 80px black bold Arial on top of the page, if it makes people use it ;)

As long as people can find you on search engines, they will come. But from there on, you have to guide them to use your service instead of expecting them to do all the work.

[terryjsmith]

This. The most important part of any project I've been involved with has been showing it to other people. You can see how they are going to try to use the product and what you might be missing. Even more so, I've had people I show it to come up with better tag lines and branding than I ever could have myself.