Basically, I think that Karka Shells are a good item to invest in right now. Allow me to explain why, and remember, we are not gambling today.
There is potential for Arenanet to utilize the Karka Shells in some way, especially because these creatures are already present in and around Southsun cove. They could be used for new crafting recipes (although not armor) for example. Even if Karak Shells aren't used at all, odds are they will rise in price up to the event's release date, as so many people will have the same ideas as I have had. Even now, the price is starting to rise on these shells. However, speculating is just gambling without understanding the market.
These Shells maintain much of their value for two very important reasons:
1. Karka's are annoying to kill. This decreases supply.
2. Karka Shells are valuable crafting components for items players want at max level. This increases demand.
Even if you get 'stuck' with Karka shells you will be able to sell them slowly or in piles due to their demand.
At the moment, prices are rising and I found it very difficult to purchase these shells with buy orders. I gave up on the small money it would have saved and bought a few stacks as buy outs. I recommend that you buy at least a small number (25-50) to get your feet wet as a smart speculator. If things start to turn south, slowly offload your inventory of Karka Shells back into the economy. Worse case scenario you might take a small loss, but odds are by the time you sell these items will have doubled in price from speculation alone.