In a change that didn't quite make the 2.1.3 patch notes, the behavior of in-game mail has been tweaked in order to help ensure that monetary transactions made via mail are legitimate. To give Blizzard more time to check mail-based money transfers, mail sent with coins attached will now take an hour to arrive (yep, just like all other mail). Now, before you get upset about this, let me add that transfers of both money and items between characters on the same account will remain/become instant. So you can shuffle things around between alts and bank characters faster than ever -- this change only applies if you're sending coin through the mail to other players.
Though I doubt this change will stop in-game gold-selling completely, it's good to see Blizzard continuing to be proactive about the issue.