Thought about this before...
What if the classifieds section was an extra? Members can't use it unless they register to that as well. In order to register you must fill in details - address, home phone no, mobile no - which are visible to people you've got a transaction open with.
I agree with the other post back a few pages, all items should be sent tracked. I send the tracking number the minute I get it, this lets the person recieving know firstly you've posted it, and secondly, when you posted it.
I ask people I'm buying from to do this. It's disappointing when they don't do it, but I've not had anything go wrong yet. It's peace of mind.