The vast # of users of this site do not participate in the EE and many of us have no desire for ARFCOM to be sending us emails every time someone sends us an IM.
If such were to happen, folks would downgrade their memberships to have smaller IM mailboxes that would rapidly fill up and thus quit sending emails, or just block or unsubscribe from notification emails.
Add a rule in your postings that if IMs are not answered in X# of days you'll relist the item?