IF OBJECT_ID('TempDB..#tblInv','U') IS NOT NULL
DROP TABLE #tblInv

CREATE TABLE #tblInv
(
ID INT ,
PartNo VARCHAR(10) NOT NULL,
TranType VARCHAR(7) NOT NULL,
Quantity INT NOT NULL
)

insert into #tblInv(id,partno, TranType,quantity)

select 1, 'A', 'buy',  50 union all
select 2,'A', 'buy',  50 union all
select 3,'A', 'buy',  50 union all
select 4,'A', 'buy',  50 union all
select 5,'A', 'buy',  100 union all
select 6,'A', 'sell',  150 union all
select 7,'A', 'sell',  100 union all
select 8,'A', 'buy',  500 union all
select 9,'A', 'sell',  450 union all
select 10,'A' ,'buy',  450 union all
select 11,'A', 'sell',  550 union all
select 12,'B', 'buy',  300 union all
select 13,'B', 'sell',  200 union all
select 14,'B' ,'buy',  200 union all
select 15,'B', 'sell',  250 union all
select 16, 'B', 'sell',  250

select * from #tblInv

IF OBJECT_ID('TempDB..#tblInv','U') IS NOT NULL
DROP TABLE #tblInv

CREATE TABLE #tblInv
(
ID INT ,
PartNo VARCHAR(10) NOT NULL,
TranType VARCHAR(7) NOT NULL,
Quantity INT NOT NULL
)

insert into #tblInv(id,partno, TranType,quantity)

select 1, 'A', 'buy',  50 union all
select 2,'A', 'buy',  50 union all
select 3,'A', 'buy',  50 union all
select 4,'A', 'buy',  50 union all
select 5,'A', 'buy',  100 union all
select 6,'A', 'sell',  150 union all
select 7,'A', 'sell',  100 union all
select 8,'A', 'buy',  500 union all
select 9,'A', 'sell',  450 union all
select 10,'A' ,'buy',  450 union all
select 11,'A', 'sell',  550 union all
select 12,'B', 'buy',  300 union all
select 13,'B', 'sell',  200 union all
select 14,'B' ,'buy',  200 union all
select 15,'B', 'sell',  250 union all
select 16, 'B', 'sell',  250

select * from #tblInv

SELECT	COALESCE(b.id,s.id) AS id, COALESCE(b.id,0) AS bid, COALESCE(b.partno,s.partno) AS partno,
		COALESCE(b.TranType,s.TranType) AS TranType, COALESCE(b.quantity,0) AS bquantity,
		COALESCE(s.id,0) AS [sid], COALESCE(s.quantity,0) AS squantity
FROM	(
			SELECT	ROW_NUMBER() OVER( PARTITION BY partno ORDER BY id ) AS rowid, *
			FROM	#tblInv
			WHERE	TranType = 'buy'
		) b
		FULL OUTER JOIN
		(
			SELECT	ROW_NUMBER() OVER( PARTITION BY partno ORDER BY id ) AS rowid, *
			FROM	#tblInv
			WHERE	TranType = 'sell'
		) s ON b.partno = s.partno AND b.rowid = s.rowid

IF OBJECT_ID('TempDB..#tblInv','U') IS NOT NULL
DROP TABLE #tblInv

CREATE TABLE #tblInv
(
ID         INT,
PartNo         VARCHAR(10) NOT NULL,
TranType   VARCHAR(7) NOT NULL,
Quantity   INT NOT NULL,
PRIMARY KEY CLUSTERED(PartNo, TranType, ID),
RN     INT,
BSOrder        INT
)

INSERT INTO #tblInv(id,partno, TranType,quantity)

SELECT 1, 'A', 'buy',  50 UNION ALL
SELECT 2,'A', 'buy',  50 UNION ALL
SELECT 3,'A', 'buy',  50 UNION ALL
SELECT 4,'A', 'buy',  50 UNION ALL
SELECT 5,'A', 'buy',  100 UNION ALL
SELECT 6,'A', 'sell',  150 UNION ALL
SELECT 7,'A', 'sell',  100 UNION ALL
SELECT 8,'A', 'buy',  500 UNION ALL
SELECT 9,'A', 'sell',  450 UNION ALL
SELECT 10,'A' ,'buy',  450 UNION ALL
SELECT 11,'A', 'sell',  550 UNION ALL
SELECT 12,'B', 'buy',  300 UNION ALL
SELECT 13,'B', 'sell',  200 UNION ALL
SELECT 14,'B' ,'buy',  200 UNION ALL
SELECT 15,'B', 'sell',  250 UNION ALL
SELECT 16, 'B', 'sell',  250

DECLARE @RN         INT, 
   @BSOrder    INT,
   @ID         INT,
   @LastPart   VARCHAR(10),
   @LastType   VARCHAR(7)
SET @RN = 0
SET @BSOrder = 1

UPDATE #tblInv
SET    @RN = RN = @RN + 1,
   @BSOrder = BSOrder = CASE WHEN @LastPart = PartNo AND @LastType = TranType THEN @BSOrder + 1 ELSE 1 END,
   @LastPart = PartNo,
   @LastType = TranType,
   @ID = [ID] -- Anchor
FROM #tblInv WITH (INDEX(0))

SELECT     COALESCE(B.RN, S.RN) RN, 
   ISNULL(B.[ID],0) BID,
   COALESCE(B.PartNo,S.PartNo) PartNo,
   ISNULL(B.TranType,'-') TranType,
   ISNULL(B.Quantity,0) Quantity,
   ISNULL(S.[ID],0) SID,
   ISNULL(S.Quantity,0) sQuantity
FROM   (SELECT * FROM #tblInv WHERE TranType = 'buy') B
   FULL OUTER JOIN
   (SELECT * FROM #tblInv WHERE TranType = 'sell') S
   ON B.PartNo = S.PartNo AND B.BSOrder = S.BSOrder
ORDER BY COALESCE(B.RN, S.RN)