Play around with the time sliders on the bottom of the graph at spidy and you'll notice the spike always tends to be a few days before Halloween starts. You'll also notice that, over time, there is a definite upwards trend of what people are willing to pay. So the best time to sell would probably be a little less than a year for now; the question is whether you'll want the stacks of these to take up valuable inventory space that long.
I've never heard of a case of items suddenly becoming account bound if they were previously tradeable. These bags are containers, so they won't become "unusable" either - you can always open them, though what's inside may become less useful after Halloween (or more annoying, as you end up with all these items taking up more inventory space after you've done your post-Halloween cleanup).