An alternate solution to this that wouldn't break the game (I can think of a number of coherent arguments as to why trading seals is a terrible idea, mostly having to do with monopolisation of content) would be to add an NPC like Ghanraam or the beastman seal NPC who would sit in Jeuno and hold all of our unused seals, cards, stones, and so on for us. This way we could stockpile seals at our leisure without having to worry about storage space; it'd alleviate one of the worst issues with seals without adding in the capacity for monopolisation.