If I were to start selling a stock footage pack, would it be considered illegal to use and effect created within after effects? For instance, creating a meteorite using particle world, and using that in what I am selling?
That should not be a problem at all, Evan. Anything you create using a plugin is your intellectual property. Where you would run into problems is if you try to sell effects presets which were created by someone else as part of their collection (for example some of the Red Giant Particular collections). What you create, however, is yours to use/sell/or do as you wish with.
As a matter of fact, I see "effects" footage being sold on Pond5 and other sites which is just the basic settings of the plugin (quite a few built in particle system effects) with little or no creative put into it. Not much more than set up on a solid, and rendered. I don't know whether any of these "base" effects are selling, but I'd sure be surprised if they are - ten minutes with AE could create many of them.