If you're playing on Icecrown and you have ever tried to sell a Stranglekelp on the auction house, you've probably noticed the absurd price. They regularly sell for 15g per Stranglekelp (300g stack) and sometimes even more. So, its of significant value to someone - but who?
I personally don't use even a quarter of the Stranglekelps I pick up because I just can't fish out that many Oily Blackmouths to make Free Action Potions. Some players might be buying both the Stranglekelps and Oily Blackmouths to make those potions but even that has its limits.
My best guess is that fish botters make up the majority of players who buy Stranglekelp stacks in large amounts so selling them feels a little dirty.
As you say they are very valuable for Free Action Potions. I don't think there's any more to it.
Yes, but for every FAP you only need 1 Stranglekelp and 4 Oily Blackmouths. The kelps are much easier to get than fish (and I know this too well from personal experience because I level fishing to 350 on every alt in areas where Oily Blackmouth is supposed to have the highest drop rates) so why are the Stranglekelp stacks equal in value to Oily Blackmouth stacks, which also sell for about 300g per stack and you need 4 times as many for a single potion?
At this time fishbotters seem to be nuked hard, there is no 50 pages of each fish for almost vendor price anymore.
Stranglekelps are valuable because people who buy FAP are not pinching pennies, people who spend time collecting them, running around for hours can charge up price for their effort, and people who flip them into FAP by purely buying reagents - make a nice profit also.
And one more thing, players who simply flip AH, they will buy up any cheaper ones and post them back for those 15g. Low quantity market and low deposit costs. It's easy market to keep up prices sky high.