The end is nigh. No more MK dungeon farming - still some time to stock up on mats and aim for achievements.
I promise, as unlikely as it seems to you - after all you're a smart player who follows the markets so much so that you head to outside resources (like this site!) - the majority of players won't have thought as far ahead as to realise that there will be recipes after the event and no materials for them 'til now. Some won't even realise until it's much, much too late.
How have you stocked up on so far? I note that candy corn is on the rise already. Not by much but this will continue as people realise it's going away.
Don't get nervous and list your entire stock of Halloween mats when you see a particular material rise to a little bit more than you paid. A lot of investment-dabblers will do this. This will mean you'll see prices reaching up to plateaus where at random they'll fall. This will be a personal decision by a mats investor "I want to make sure I get money back, at this rate rather than nothing". It's fear on their part but may actually start knee jerk reactions on the part of other investors who think it's a crash.
You want to think calmly, check around and establish two things if you can before you list some (never all at once, never!) of your stock for sale.
1) Has any news been released to the community that indicates stocked GW2 Halloween mats will lose value.
2) Has any major site speculated that NOW is the "peak" time to sell stocked guild wars Halloween mats?
If you can eliminate these two to the best of your ability then you're looking - almost 100% - at a heavy investor or a series of medium investors shedding their stock. Maybe they really need to buy something and are liquidating. Maybe they're panicking. Either way you don't need to follow. Be patient.
Theoretically there doesn't need to be a peak for a material that won't reenter the economy. This isn't strictly true but bear that in mind if you start to feel the hole drop out of your breeches.