It could be that you're overstepping the value of your trade item, but that's not too likely. I think the real reason is, the things you're asking for in exchange for it are probably just as difficult to get, and that generally means it's much more difficult to part with. I've done a few midlevel trades myself, and I still can't bring myself to get rid of my trusty Soul Eater, even though it's generally looked down on by other high-level players. It's sort of an attachment to scythes. I like the grim reaper thing going on.
But anyways, in summary, you might want to try to coerce people you're trading with by dropping a few extra goodies into the trade, or by taking them into the game and showing them just exactly how tough the Flame Visit is.