Urag gro-Shub won't buy the Elder Scroll
Solution 1:
The Elder Scrolls are Quest items, therefor can't be sold until ALL related quests are completed. So you got to check if the following quests are done:
- Elder Knowledge
- Alduin's Bane
- Discerning the Transmundane
And if Dawnguard is installed:
- Scroll Scouting
- Unseen Visions
Also because it is a quest item it won't weigh anything: Do quest items weigh anything? as stated you can try it out by placing everything you own in a chest and check your weight.