I noticed the GT5 Marketplace is a bit cluterred with completed trades, is it possible to set it so that setting the tag of the thread to "Completed" auto-locks it or is that not possible in vBulletin?
Unfortunately, there's no technical way to "hide" completed threads from the forum. However, in addition to filtering the topics by the other prefixes, you can show threads with either the Wanted or Available prefixes using the advanced search page (just hold down the CTRL button to select more than one prefix).