Instructions
Search your Pokemon collection, looking for any cards you want to sell. The most valuable cards are the rare ones that are produced in limited numbers.
Check the value of those cards on a specialized Pokemon trader website like Pokecorner.com. These sites publish values for cards, listed by the name of the Pokemon on the card and the edition that it came from.
Sell your rare Pokemon cards through trader websites if you like the prices offered. Just follow the instructions provided on the sites. But be careful; you may be able to get a better price through other means.
Take your Pokemon cards to a local coin and card collecting shop to get pricing. See if they are interested in buying the cards from you.
List the rare cards for sale on eBay, craigslist or a similar auction site. You can set a minimum price, then auction them. You may make much more money than by selling the Pokemon cards outright.