When you have created the CTXCAT index on the AUCTION table, you can query this index with the CATSEARCH operator.
First set the output format to make the output readable:
COLUMN title FORMAT a40;
Now run the query:
SELECT title, price FROM auction WHERE CATSEARCH(title, 'CAMERA', 'order by price')> 0;
TITLE                PRICE
--------------- ----------
PENTAX CAMERA          200
CANON CAMERA           250
OLYMPUS CAMERA         300
NIKON CAMERA           400
SELECT title, price FROM auction WHERE CATSEARCH(title, 'CAMERA', 
     'price <= 300')>0;
TITLE                PRICE
--------------- ----------
PENTAX CAMERA          200
CANON CAMERA           250
OLYMPUS CAMERA         300