I set up hosting and email for each domain with a catch-all address for each. Then I set up an auto-responder for the catch-all so that way every email that comes to the domain sends an auto reply:
SUBJECT: Returned Mail from XYZ.com
Your mail has been refused for the following reason(s):
This domain has been acquired by a new owner and no longer accepts mail at this address.
Please remove all email addresses from your list or address book for the domain XYZ.com
If you feel you have received this message in error or need to contact the domain owner, please use the contact form at:
http://XYZ.com/contact.html
This way people who are interested in buying can use the contact form - the spammers don't usually bother unless it is some schmuck who has nothing better to do than send out individual Viagra adverts.
This method also increases traffic to the domain, because people get curious at the mention of a "new owner".