You're forgetting that not everyone reads these posts on google groups
via a web browser.
Many read these postings as email, just like a listserv. Deleting the
post is never noticed by these readers: they got their message already,
it's in their inbox, and deleting the message from the google group
doesn't send them anything and doesn't remove the message from their
inbox. On the other hand, sending a message that the item is sold lets
them know it's been sold.
--
Steve Palincsar
Alexandria, Virginia
USA