Some may ask themselves, if it's better to sell or to open these bags. The answer is sort of subjective, but I'll try to state my thoughts and would like to invite others to add their opinions about it here.
https://wiki.guildwars2.com/wiki/Trick-or-Treat_Bag
The prices after this first week are continuing to drop and I'd say you have to ask yourself what you want out of it.
It's a great source of gold, if you need it, since they can be farmed easily with any profession all day long and sell like hot cakes on the tp.
Opening them floods your inventory with plastic fangs, chattering skulls, nougat centers, candy corns, various halloween tonics some finishers, rotten eggs and toilet paper.
As of writing this post, a stack of 250 bags sells for roughly 7g 80s on the tp.
If you are after Candy Corn Cobs, my advise would be to sell the bags and buy the cobs directly. Opening a whole stack of bags, leaves you with give or take 1000 candy corns, which can be traded into one Cob, however on the TP a cob goes for 4g 30~90s currently. Thus you can get almost 2 cobs for a single stack of bags.
They also have a rare chance to drop items like Vassago, Onus etc as well as T6 materials. Also, they can drop Courtly Weapon chests, which are account bound and contain a choice of one of 12 weapons with a unique skin you can get only from this chest or from guaranteed unlock tickets.
https://wiki.guildwars2.com/wiki/Courtly_Weapon_Chest
Lastly there is a super rare chance to get infusions, some selling for a pretty penny, but usually you'll need to open tens of thousand of bags just to get one.
tl;dr: don't care about the weapon skins and super rare chance for infusions? sell the bags!
need candy corn COBS? sell the bags and buy them directly.