When you talk to a trader to look at his wares, the tooltip of equipment he is selling only refers to the profciencies of the character who initiated the conversation with the trader and clicking through your party member's inventory doesn't adjust the tooltip.

So, for example, you start a trade with a wizard and switch on the trade window to Shadowheart and hover over a shield, the tooltip still says "no proficiency" because the tooltip still refers to the wizard and not to Shadowheart.
You have to end the trade and talk to the trader with Shadowheart again for the tooltip to actually show her proficiencies. Same goes with every other member of the party.