eBay. But sell the really rare ones individually, as some can really go for a lot. I have a Black Charizard card iirc that has a $79.90 equivalent on eBay. I need to sell all my old ones.
Well there is a site I used to frequent by the name of pokeorder.com, they sell individual cards there and their prices are a good estimate of how much they're worth. But cut the price in half 'cause it's eBay and people can bid on them anyway.
Problem is, it would take hours of re-organizing. I have so many energies (used them in competitions I won, btw) that I would have to take them out and move them somewhere else.