Ok this is what I'd do - and I'm assuming you can make modifications to
the spreadsheet - ie inserting columns -
I'd insert a column between B and C and enter in a formula that would
combine column A and column B ( seller and product for Jan)

=+A2&" "&B2

Then insert a column between the NEW column F and G and enter a formula
that would combine column E and F ( seller and product for Feb)

=+E2&" "&F2

Now you can write a VLOOKUP formula to see if you find a match on
CUSTOMER/ITEM from this months (Feb) compared to last months


=IF(ISERROR(VLOOKUP(G2,$C$2:$D$21,2,FALSE)),"new", H2-(VLOOKUP(G2,$C$2:$D$21,2,FALSE)))


This will give you NEW if a match is not found, or calculate the
difference between last months and this months.

here are the columns and items I have

A B C D
E F G H
I
customer item customer/item comb Jan Price customer item customer/item
comb Feb Price Formula

One issue - you'll need to make sure that you have the exact spelling
in each month for both name and item or it won't find the item
correctly.


Hope this helps send you in the right direction.