Disk space is practically a non-issue. I would be surprised if the entire PF database, minus photo attachments, takes up more than about 10GB; mySQL compression algorithms are pretty good and words crunch up really well.
As for confusion of searching, you can specify a date cut-off on the
advanced search page so that only threads/posts active within a certain time range show up.
Having a historical record however does serve purpose. It provides a tool by which buyers and sellers can get an idea of what price a given item typically sells and depreciation trends. The feedback system also depends on sales threads to exist so people looking at feedback can see what occurred to a degree between buyers and sellers. Additionally, the search engines eat up forum threads and posts and by keeping as many of them alive as possible that means this site gets better ranking in the search engines bringing more members with it.