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.
There could be a jutsu that allows for instant teleportation across long distances. This jutsu might be a new discovery in the fanfiction world, perhaps created by a character who has been experimenting with space - time ninjutsu. They might use it to quickly get in and out of battles or reach far - off places in a blink.
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.
Maybe Sasuke. Although they are rivals, they also have a deep bond. Sasuke might have come across a unique jutsu during his travels and decide to teach it to Naruto so that they can both become stronger and better protect their village.
He might learn a time - traveling jutsu. This would be really cool as it could allow him to go back and change certain events in the Naruto world, or gain knowledge from the past to use in the present battles.
A 'Solar - Flare Rasengan'. Naruto could combine his wind - style chakra with some kind of solar - based energy he discovers. This jutsu would be like a super - charged Rasengan that emits a blinding light as well as powerful wind - based damage.
He could potentially learn the Flying Thunder God Technique. It's a very powerful jutsu that Minato was famous for. Naruto's already fast with his Sage Mode and Nine - Tails Chakra, and this would make him even more formidable in battle.
He could practice in secret locations where no one can see him. For example, in an abandoned cave or a hidden part of the forest. That way, no one will know about the new jutsu he's developing.
Well, in certain fanfic scenarios, Naruto could learn paper jutsu as a result of some experimental training. For example, he might be trying to combine different elements of his existing jutsu and accidentally discovers a way to manipulate paper. It could be that his chakra takes on a new form during a particularly intense training session and he realizes he can control paper with it.