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.