Have you also tried the Calamity Salvager npcs found in each city? In a patch earlier in the year they were altered to provide quested and event items for resale that you had already attained but lost (for instance, accidentally throwing away). As the Blazefire Saber was event gear, it's likely when you purchased it during the event it 'flagged' it in your inventory, hence it would then be available for purchase again at the Calamity Salvager. This is an assumption on my part though as I've never used them before. Look for a npc wearing a distinctive grey and red tunic and the name 'Calamity Salvager'.