I set up a automotive classifieds for a friend on a 3.8 site using Easy Forms by bananalive, Thread Thumbnails by Null Parameter, Mark as Sold by venom2124 and a few conditionals. I also edited the forumdisplay template to hide the normal New Thread button & added Create Ad button just for the classifieds forums.
This is a
link to directions using 3.8,,, Everything I used to create his classifieds can also be found in 4.0, use it as a quide, yes its simple but what more do you really need?