Hey there!
My brother was just having the same problem. Here's what you do - Remember where you trained for your mount in the Hinterlands? Well saddle up and head back to the Redcliff Farms in the Hinterlands. Right next to where his daughter runs the stables and races, you will see a house. Knock, wipe thy feet and talk to Master Dennet. The Inquisition would sure enjoy to have a man tender to the stables at Skyhold, no? Ask him to join the Inquisition! That's all! He will then be back at Skyhold tending to your stables and the Buy/Sell sign will be active. Note: You must complete any pre-quests before you can ask him via the dialog option. I'm assuming you've already done this. If not, do the couple quests he has and then ask him to join.
Master Dennet does such a good job you don't even smell the poo.
Kind regards,
KONCEPT
