If you want to set the values of multiple variables in a single statement, you can use the SELECT statement:
SELECT @ItemName = item_name,
       @Category = category

Instead of an IF statement, why not a CASE expression?

SELECT oi.item_name
       CASE oi.item_name WHEN 'Pen' THEN 'Accessories'
                         WHEN 'Pencil' THEN 'Accessories'
                         WHEN 'Cable'...

Could you perhaps format your code so that it's readable please? Having a CASE expression with each part on a separate line with 3 blank lines inbetween makes in completely...

This isn't editing your code, but I've had a similar problem with an SSIS task that needs to be called from the our website. When the task is kicked off,...

SQL Server 2017 is in CTP at the moment, I'm not sure it was be advisable to upgrade your current version to a test product. You would be better setting...

You say your have variables, but these are column names in your query. Variables start with an @ at the beginning.

You could use a CTE to do this...

As Jeff suggested, a TVF would be a much better option than a Scalar funciton for performance. So you could do something like this:
CREATE FUNCTION dbo.LastBuyer_tvf (@SC1...

You're using your CASE expression incorrectly.

Try:
ALTER FUNCTION [dbo].[LastBuyer]
(@SC1 varchar(30), @CIE varchar(1))
RETURNS VARCHAR(3)
AS
BEGIN
DECLARE @LB VARCHAR(3)
SELECT @LB = CASE...

Just to point to put, that isn't exactly what you asked. That query returns a list of logins that have their default Schema set to dbo on the current database.

Do you mean?
SELECT SYSTEM_USER AS ServerLogin, CURRENT_USER AS DatabaseUser;

Perhaps something like:
CREATE TABLE #Sample
  (Org_ID varchar(20),
  Amount decimal(12,2),
  date date); --That is not confusing... </sarcasm> 😉
GO

INSERT INTO #Sample