PL/SQL function while loop returning 1 row only

The function you wrote was supposed to return One row only. And also, you had a return in your LOOP. So after first iteration the control is returned back.(leaving the cursor opened for ever)

Create a TYPE of SQL Object

create type my_numbers as table of NUMBER;
/

FUNCTION returning a table!

CREATE OR REPLACE FUNCTION  purchased(cId IN NUMBER) 
RETURN my_numbers
IS  
    product VARCHAR2(45); 
    I NUMBER;
    v_my_list my_numbers := my_numbers();
    CURSOR CursorRow IS 
    SELECT p# 
    FROM customer c  
    LEFT OUTER JOIN pbasket pb 
    ON c.c# = pb.c# 
    LEFT OUTER JOIN pp pp 
    ON pb.whenfinalised = pp.whenfinalised
    WHERE c.c# = cId;
    CurrentPos CursorRow%ROWTYPE; 
BEGIN
    OPEN CursorRow;  
    I := 1;
    FETCH CursorRow into CurrentPos;
    LOOP
    EXIT WHEN CursorRow%NOTFOUND    
       v_my_list.EXTEND;
       v_my_list(I) := CurrentPos.p#;   
       I := I + 1;
    END LOOP;
    CLOSE CursorRow;
    RETURN v_my_list;
END purchased;
/

Final SELECT Query:

select  * FROM TABLE(CAST(purchased(100) as my_numbers));

We can also use Pipelined functions (Slight modification needed) for performance over Large resultsets!