how put result of a select query to a function in postgresql

If you want to use scalar subqueries (that is, subqueries that return one value), then each needs their own parentheses:

select get_open_profit( (select sum_buy_trades
                         from results
                         order by sum_buy_trades
                         limit 1
                        ),
                        (select total_avg_buy
                         from results
                         order by sum_buy_trades
                         limit 1
                        )
                       );

In this case, though, the query might be more naturally written as:

select get_open_profit( r.sum_buy_trades, r.total_avg_buy )
from (select sum_buy_trades, total_avg_buy
      from results
      order by sum_buy_trades
      limit 1
     ) r;