Will a previously installed operating system work on my laptop?


I have OS X v 10.4 currently on my MacBook Pro. I need to upgrade to 10.5. My dad wants to buy it for me (probably on eBay), but wants to know if he can buy a used copy of it or if it needs to be a new unopened version. If a copy has already been installed on someone else's computer, is it possible to also install it on mine if I buy it from them? Seems safer to just buy an unopened version, but they are quite expensive.
Any insight you can give me would be greatly appreciated.
The software comes on a DVD.
It will install without knowing if it has been used before.
The software cannot, and does not keep track of how many times it is installed.
There are no special "codes", or registration numbers to type in, so:
Nothing prevents you from installing 10.5 on any computer that supports that version of OS X.

After installing, the new user setup will give you the opportunity to connect to Apple's servers for registration, which you can choose to ignore. That will not prevent use, and is not required.

However, if you do have a Macbook Pro (and NOT a PowerBook), then you can also upgrade to Snow Leopard (OS X 10.6), without needing to upgrade through 10.5 first). And 10.6 is available directly from Apple, and will be less than $20 (including shipping - which is free). You will find the 10.5 is still more (and can be MUCH more) than $20.
Snow Leopard is here: http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
Oh, wow. Thanks. I actually already own 10.6, but thought I had to install 10.5 first. If I can just go straight to 10.6 that will save me a lot of money and time. Thank you.
If you want to install MacOS X 10.6, then there is no need to install MacOS X 10.5 first.

Caveat: Be sure that your DVD has the large X and is not a grey disc. The grey discs are System Restore discs that unscrupulous people sell to naïve Mac users. Each System Restore disc is intended to return a specific model Mac to its as-shipped state. It can be used in that specific model and no other.