Quest Items weigh 0 despite what their proper weight is recorded as. The weight only comes into play once they lose their quest item properties (and become dropable). If you completed for the Dawnguard you can sell (or give) the two new scrolls to the Moth Priest, but for the Vampire Lord side you're stuck with the two scrolls. You can't sell the Dragon scroll to the Orc if you've bought it back from him once, which is really annoying.
If you're on the PC you can just install a mod which makes them dropable, but console players just have to bear with them.