Logic Pro 9 asking for original serial number

The cheapest would be Logic Pro 9 from the App Store but you will have to fully uninstall Logic Studio (including removing the ProApps ID files) first otherwise it will again, ask you for the various serial numbers required.

However, note: only the Logic Pro app itself will then work and none of the other Logic Studio applications... such as Waveburner/Soundtrack Pro because they are not part of the product now sold on the App Store.

The boxed sets of Logic Studio 2 (Logic Pro 9) and Logic Studio 1 (Logic Pro 8) for that matter... can often be found for about or just slightly more.. than the same price as the Logic Pro 9 version from the App store, these days online..but make sure that if you choose to purchase Logic Studio 2...you purchase the Full version and not the upgrade version otherwise you will be back in the same boat once more.

Given you only need the serial you don't need to be concerned about what OS X version you are running.. (because Logic Studio 1/Logic Pro 8 will not normally install on a Lion/Mountain Lion based Mac due to the lack of PPC support these days)

As an aside.. The Logic Pro 8 Full serial can normally be found on a sticker that was attached to the back of one of booklets/sheets that came with the boxed set.

Finally, If you still have the recepit for the version you got off of eBay you might want to first try obtaining a replacement serial for Logic Pro 8 from Apple via this webpage

http://support.apple.com/kb/ht1861

I don't know how you had it running in the first place. If you bought a set of used disks with no number, there's no way to run that. And if you bought a new set of upgrade disks, that would only include an upgrade serial number and would also require either a number from Logic 7 or 8, or having the old dongle plugged in. The disks are the same whether it's a full version or upgrade, the difference is the kind of serial number it comes with.

You probably need to call up apple and see if they can help you out.