You can do a fresh installation of SL by using the CDs that came with your MacBook.
As for Lion, you don't have to buy it again but if you go to SL, you will have to download it again. If you have saved the lion installation file, here is what you can do:
Locate the OS X Lion installer and right-click on it. Select 'Show Package Contents'.
- Inside the 'Contents' folder, there is a 'SharedSupport' folder. Inside that is the Lion Installer. It's called 'InstallESD.dmg".
- Copy 'InstallESD.dmg' to the Desktop by clicking and dragging it while holding down the Option-key. You should see a little green plus icon if you did it right.
- Open Disk Utility. Head to the Go menu in the Finder and select 'Utilities'. Disk Utility should be in there.
- Click the burn button.
- Select 'InstallESD.dmg' from the Desktop, insert a blank 4.7GB DVD and wait. Once it's finished, you'll have a shiny new Lion install DVD.
You can now install Lion on whatever machines you like, just as if you'd purchased the install disc from your local Apple Store. Lion is expected to launch on the Mac App Store this week.