But you cannot Buy them in shops. Which makes it worth whatever the people want to sell it for. Bow-Swords were selling for like 1gp at shops, then you could sell them to people for 1-2B a long time ago, same with Firecape when it was tradable, it was 100M in the shops, and it was sold for 100-1B each
But still, you make more money out of selling at the shops, if you are the seller, you can sell 200m to shops, or 100m to a person, what would you rather? Of course the 200m.