Runeseekers are scholars who adventure throughout the land seeking 'wild runes'. While some Runeseekers use magic themselves, many just gather them in magical books and sell them to wizards or mages.
There is a difference between using magic, seeking runes, and casting runes. Magic users draw on mana, rune seekers simply seek out naturally occuring runes, and rune casters can actually use discovered runes as if they are using magic themselves. There are also pools of wild magic and mana in the world, these pools are often filled with naturally occuring runes.