Vat on exchange of goods

I went to a store (they have franchise in India) and traded my second-hand Wii for a second hand PS3.  The store gave me credit for Rs 3000 for my second hand Wii and since the second hand PS3 was for Rs 12000, I paid cash of Rs 9000.  

When I got the bill, though, I was surprised to not see any VAT on the bill.  The company's VAT # and CST # was printed on the bill but there was no mention of any VAT for the Rs 12000 or Rs 9000 that I paid.

How is that possible?  Is this a legitimate bill?  If I wanted to expense this purchase, would I be able to with this bill or do I need anything else?

Thanks!