I think this will work as well:
SELECT
    P.person,
    P.category,
    P.item,
    ISNULL(SUM(S.Qty), 0) AS Qty
FROM POSS AS P
    LEFT OUTER JOIN Sales AS S
        ON P.person = S.person
        AND P.category = S.category
        AND P.item = S.item
GROUP BY
    P.person,
    P.category,
    P.item
ORDER BY
    P.person,
    P.category,
    P.item;