What exactly is the problem? Do the items not show up in his inventory, or does he refuse to sell you any stuff?

The silver brooch apparently disappears later in the game, according to the wiki. The only reason for him not to have a backpack is if you've already reached the inventory limit of 120 (console) / 125 (PC).

My bad i thought the inventory limit was 130. Do you have link for the silver brooch ? I never saw the mention of disappearance.

https://dragonage.fandom.com/wiki/Varathorn

At the end of the inventory list.

Thanks.