How about a "delete message from inbox after sending reply" button in PMs.
It's common courtesy to respond to a PM.
Often times we just respond with a "no, sold already" or "sorry, I have a better offer", and then that received PM just sits in the inbox wasting space until it's clean it out.
Under the "Miscellaneous Options" can there be another check box to "delete message from inbox after sending reply". This would help people maintain the common curtesy of responding to PMs while making it easy for them to keep clutter to a minimum on the fly (as opposed to doing inbox maintenance when the box is getting full).