Your issue is your style not our product!!!
We only use as we note the xenforo style classes meaning the default style classes used against a the default xenforo style elements. There has to be a standard to support a style match. This is quite commonly known. If there is no uniform style then we would move the product down to the lowest form of integration meaning user login only.
It you look at a default xenforo forum and a default classifieds integrated with it it clearly pulls the correct normal style classes. There is no problem with the look and feel. We do not pull anything else from xenforo like we do in vb like headers footers.
If your using some custom xenforo style which might use different style class names and hard coded html around the tables none of that is going to show in classifieds. This is what I see your issue is.
I see nothing wrong with your classifieds given this scenerio. Here is a default photopost pro installed on a clean xenforo test install. I can easily install classifieds on my test site and it would look the same as you see here. Now just like your site my test site has a mock up of the xenforo header footer.
A community photo gallery - My Photo Gallery
You can edit your style matches in the xenforo.php file in the forums subdirectory of the classifieds product. You will see the style match.
For instance like a default xenforo class names we match are here
Content visible to verified customers only.
You will see if you look at your site for instance a default xenforo table header class is
nodeInfo categoryNodeInfo categoryStrip subHeading
where as your custom style uses for the sideboxes which would probally be easy for you to switch to
or you can try other style matches but this all comes down to your style not being identical to the style classes used in the default xenforo.
So basically what would happen at this point is you can match things up how you wish but if this issue is more than you just choose to buy a theme that does not match the supported xenforo theme class calls and xenforo is going to allow any style class name to be used them we will move the classification of the integration to user login only.