There is a possibility that it was an ex - shinobi who had retired from active duty. This person had amassed a collection of seals during their career and now made a living by selling them to other ninjas like Naruto.
Well, considering Naruto's own experiences and the variety of seals in the Naruto universe, he could sell communication seals. These would allow ninjas to send messages over long distances. He might also sell seals that are related to elemental jutsu, perhaps ones that enhance or modify fire, water, wind, earth or lightning techniques.
Perhaps Naruto was in a situation where he had made a deal. For example, he promised to pay for the seals in exchange for some information about a missing nin or a secret technique. He values knowledge and power, and if paying for the seals was part of getting closer to achieving his goals, he would do it.
The seals could also be a plot device for a darker storyline. Maybe they were placed by an unknown enemy to control Naruto or use him as a weapon. In some fanfictions, the seals start to affect Naruto's mental state, causing him to have inner struggles between his normal self and the influence of the seals.
I'm not sure exactly as there could be many different interpretations. It might be about new types of seals in the Naruto universe and how they affect the fate of the characters. Maybe it focuses on a character who discovers a powerful seal that changes the course of their life or the entire ninja world.
There are many 'Naruto Seals' fanfictions out there. For example, some might explore how different clans in Naruto use unique seals. A fanfic could center around a young ninja who is a prodigy in seal creation, and his journey to master all types of seals. There are also fanfictions that involve Naruto himself learning about a new and dangerous seal that he must keep from falling into the wrong hands.
The idea of Naruto sealing the Juubi is quite common in Naruto fanfictions. If Kurotsuchi is involved, it could be that the author wants to explore a different kind of relationship. For example, Kurotsuchi could be from a different village and her interaction with Naruto during this momentous event could lead to a new alliance or at least a better understanding between their respective villages. Also, the sealing of the Juubi by Naruto is no easy feat. It would require a great deal of chakra control and knowledge of sealing jutsus. Kurotsuchi, with her own skills in earth - based techniques, might be able to contribute in unexpected ways, like creating a stable environment for the sealing to take place.
One - handed seals might allow for more complex jutsu combinations. Since the hands are freer, a ninja could potentially chain together different jutsu more seamlessly. For example, a fire - style jutsu followed immediately by a wind - style jutsu to enhance the attack. This would change the power dynamics as it gives the user more versatility in combat.
Obviously, Naruto is one of the main characters as the title implies he is the one sealing the Kyuubi. Also, the Kyuubi itself is a main character in a sense. It has a major role in the story as it is the entity being sealed.