Buying PC games used is pretty much ALWAYS a bad idea in this day and age.
If you wait you can get them for similar prices on digital sales on steam, GOG etc. And you will then be able to download the game as much as you want.
What version of the game did you get exactly? For North America for example, the latest expansion Rebellion has never had a physical release.
You could try to email Stardock support with a photo of your purchase/key and hope they'll be nice and give you a new key. They don't have to do that however.
In addition, I do not believe Stardock physical releases come with any on disc DRM. So you can at least play version 1.0 of the game without issue, unlike some other companies.