This has happened to a couple of people I know, and then myself yesterday.
It’s too easy to purchase the wrong item from Dungeon/Karma vendors. No refunds by Anet are offered to players who make a mistake, “double click” or lag out and buy.
-Add a confirmation box to the selection that requires an additional click.
-Don’t allow players to buy items their character can’t equip.
-Allow players the chance to sell the item back to the original vendor for the same price within 15 minutes of purchase.