IF is a PL/SQL construct. If you are executing a query, you are using SQL not PL/SQL.
In SQL, you can use a CASE statement in the query itself
SELECT DISTINCT a.item,
(CASE WHEN b.salesman = 'VIKKIE'
THEN 'ICKY'
ELSE b.salesman
END),
NVL(a.manufacturer,'Not Set') Manufacturer
FROM inv_items a,
arv_sales b
WHERE a.co = '100'
AND a.co = b.co
AND A.ITEM_KEY = b.item_key
AND a.item LIKE 'BX%'
AND b.salesman in ('01','15')
AND trans_date BETWEEN to_date('010113','mmddrr')
and to_date('011713','mmddrr')
ORDER BY a.item
Since you aren't doing any aggregation, you don't want a GROUP BY in your query. Are you really sure that you need the DISTINCT? People often throw that in haphazardly or add it when they are missing a join condition rather than considering whether it is really necessary to do the extra work to identify and remove duplicates.