Selling Leopard upgrade disk?

If it came with the system its considered OEM which means you wouldn't be able to transfer it legally.

 

Did you buy that leopard family pack for another mac? If so, then the one that came with your iMac will not work with it. The ones that come with a mac are tailored for that model I believe. Now, if you wanted to sell the one that came with the iMac, then you will have to find someone who has the same model as you, but this would be illegal.

 

I've seen a few on eBay for sale, they never seem to fetch very much so I wonder if it's really worth the hassle of listing it etc. I'm in the same boat as you, I brought a Mac Pro which came with Tiger, then I purchased a family for Leopard, then I got a Mac Book Pro which came with an OEM disk for Leopard and now my sister has an iMac with Leopard.... I wouldn't be inclined to bother if I was you my friend.