I'll try to explain using your original code.

USE [OutletRetail]

GO

/****** Object: StoredProcedure [Outlet].[sp_UpdateProductStatus] Script Date: 01/16/2014 19:58:47 ******/

SET ANSI_NULLS ON

GO

SET QUOTED_IDENTIFIER OFF

GO

--Updates Product status codes to Available if NULL

ALTER PROCEDURE [Outlet].[sp_UpdateProductStatus]

AS

DECLARE @strProductNo varchar(20)

--You declare 2 SellerNo.

--The first one is to fetch from the cursor

DECLARE @strSellerNo1 varchar(10)

--and the second one is to check for a different value for SellerNo

DECLARE @strSellerNo2 varchar(10)

DECLARE UpdateProductCursor CURSOR FOR

--This is your cursor definition, if you run this query, you'll know the values that will be used.

--Be aware that cursors defined with the default options are dynamic and affected by updates to the data.

SELECT Product_No, Seller_No

FROM Outlet.tblProductMaster

WHERE Product_Status IS NULL

OPEN UpdateProductCursor

--This retrieves the first values of the cursor.

--At the end of your while cycle, you'll have the same statement to navigate through the cursor.

FETCH NEXT FROM UpdateProductCursor INTO @strProduct_No, @strSellerNo1

WHILE @@FETCH_STATUS = 0

BEGIN

SET @strSellerNo2 = NULL

SELECT @strSellerNo2 = Seller_No

FROM Outlet.tblProductMaster

WHERE Product_No = @strProductNo

--Here, you're looking for rows with the same product_No but different Seller_No

--If there's more than one value for Seller_No for the same product, the variable will have a value assigned

--If not, it will stay NULL

AND Seller_No <> @strSellerNo1

AND Product_Status = 'Available'

--This is a validation to run the update only if the product has a single SellerNo.

IF (@strSellerNo2 IS NULL)

BEGIN

UPDATE Outlet.tblProductMaster

SET Product_Status = 'Available'

WHERE Product_No = @strProductNo

AND Seller_No = @strSellerNo1

END

I would still recommend that you post the complete details to improve the code and get rid of this cursor.