Well, could always open an Ebay store, probably the easiest and safest route. But I dunno about a website in general, consider there are already a lot out there dealing in TCGs in general, one focused solely on Pokemon is gonna be hard to crack unless you're willing to undercut them, and even then profit margins probably are gonna be razor thin as those sites already operate likely close to that edge too, especially in terms of commons and uncommons.
Short version, if you're looking to get rich off it, not gonna happen unless you get some ungodly luck and can sell nothing but like Mewtwo EXs and other popular cards while never getting any bad cards that won't sell much.