Some of the problem, I think, comes down to users who post modules and don't make a selection as to whether a module is "currently available", "concept/prototype", or "discontinued". If people would show a lot more discipline about doing that when posting something new, then it would be easy to use that existing filter to deselect the unwanted entries.
Using the "currently available" filter in searches does help some...but then one runs into the issue of modules that are definitely available but not labelled as such. Again, this is a user issue as I note above.
reidv does make a good point, also...if you're posting something that was a one-off and which is only in your rig, definitely use that "unlisted" checkbox unless you actually plan on making said module(s) available for sale.