If you are installing ClassiPress on the existing WordPress site then it will of course take over the existing theme you are using. The main consideration here is that our themes make use of custom post types, and therefore when you simply replace a theme, all of the posts you have on your current site will not appear in "ads" of ClassiPress on their own. A little more about this, and how the URLs are structured are explained in this
blog post. If your current site is not actually set up as a classifieds site, and the posts will remain as "posts" and appear in the blog section where the URLs will not be effected in the same way. Thanks.