When I build a set I tend to keep the box and instructions as I noticed they add to the value of the set.
I also keep the extra parts in the box but I'm not so sure if they add actual value. My understanding is they are random so there is no way to confirm they are the original pieces that came with the set.
Is there any value in keeping them along with the set when selling?
UPDATE: Based on the comments I understand the extra parts are consistent and predictable. So I would have thought having extra parts in would make the set more valuable. In BrickLink generally the status of box, minifigs and instructions are mentioned. No mention of extra parts.
Am I correct to think it doesn't add value if we included extra parts?