I believe the only way to get Lion back onto your laptop is to update to 10.6.8 (I think that's the right version of Snow Leopard), then open the App Store and download Lion from there.
I'm not sure how you would be able to update to SL's most recent update though; maybe it'll let you bypass all the other updates and skip straight to 10.6.8.
If all else fails, go to Apple and see about wiping it and reinstalling Snow Leopard?
To make sure I understood correctly:
The Macbook had 10.7 on it when you bought it? And it originally had 10.4?
If so, then you will need to purchase Lion. The license for Lion purchased at the app store is not transferable; the seller should have erased the drive and reinstalled the original system. So, the easiest will be to purchase Snow Leopard, update it, and then go to the app store and purchase a legal copy of Lion. Each is $30 or less.