This is an example of a replacement for all the code posted to use the cursor.
UPDATE t SET
Product_Status = 'Available'
FROM Outlet.tblProductMaster t
WHERE Product_Status IS NULL
AND NOT EXISTS( SELECT *
FROM Outlet.tblProductMaster x
WHERE x.Product_No = t.Product_No
AND x.Seller_No <> t.Seller_No
AND x.Product_Status = 'Available' )
Less than a third of code lines (and I'm not counting blank lines) and should perform a lot faster.