This currently isn't allowed for forum members.  I could see the argument where this might be of some benefit in the Classified forum, but it isn't easily applied in one forum without applying it to all...and I don't want to do that.

If you don't want to reply to your own topic with "snake sold" (or what have you), you can request a moderator to change the topic title to indicate the status, say from "Columbian Boa" to "Columbian Boa - SOLD".  Moderators can also delete topics, but I'm just not a big fan of deleting them.  Moderators can also close topics to further replies.