Are you trading with the lead character (whoever started the conversation)? If you switched, did you check both inventories?
Do you have a resurrection scroll in a backpack, and the ones you buy get automatically added there?
For the actual trade mechanic;
Click a stack of resurrection scrolls, adjust the split stack window to the desired amount and transfer some to the middle right column. Then (or previous) click on any loot you want to trade with, and then hit the balance icon to even out the difference with gold (if available). Now when you hit the trade button the scrolls should be added to your inventory and your trade goods added for the merchant.