I think it will be a great idea if players can sell siege weapon blueprints on the trading post, especially the upgraded versions. My friend, playing WvW for one night, burned through approximately 100 superior siege weapons.
For an individual player, the number of skill points required for this is not sustainable. Why not make upgraded blueprints tradeable on the TP? That way, people who have excess skill points or who just have blueprints but don’t like using them can get rid of them in an easy manner, while people who enjoy WvW do not need to grind the experience to continue making superior blueprints.