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