select item_id, buyer_id, sum(amt) as buyer_all_amt
from A
where where dt >= '20180701' and dt <= '20180930' and seller_id = 123 and
item_id in (select a.item_id from
(select item_id, sum(amt) as all_amt from A
where dt >= '20180701' and dt <= '20180930' and seller_id = 123
group by item_id
order by all_amt desc
limit 1) a )
group by item_id, buyer_id
order by buyer_all_amt desc
limit 1