Pokémon prices are very subjective, first of all it depends on what card it is, then if it's stamped with the pack logo, a promo card or even if it's a staff release. Some cards may not even sell well if it's not even in rotation, meaning that the cards being used in tournaments at the given time aren't valuable, not because no one wants to buy them it's just that the players aren't looking and your going to need high value cards to sell to collectors. Pokémon cards have crazy price trends, use pricecharting.com to check the value of the cards you have. If you think you have a great card, you may want to make sure that card never even breaths air because it will depreciate if you're hoping to get it graded as a 10. Use some white gloves (like those used in church) get a clean penny sleeve and a plastic card protector, even a mat because if it does fall it won't hit anything hard, and the card won't get scratched.
EBay is a game changer when it comes to looking for anything, but you can resell and find anything anywhere, but with eBay you wont have to leave your home, at all >:)... For that fact if you want to resell Nike Air Jordans you need to find someone selling them for a lower price, right? Like in stock... you buy low and sell high. Same with Pokémon cards, you see how much there going for (well which card you can currently find) and you find get that lower price, you get it shipped to you then you resell it at the market price. Don't worry it could of happened to you or a friend, even on accident. Only because you didn't know the actual value of the card that's why you have to do the research. Don't worry there will be a graph at the bottom showing you how its all put togther.
Using pricecharting.com let you see the prices of Pokémon cards at the current time. You can check the graph of the card from ungraded to PSA 7-10. You first want to find a card in your price range. You can pick any card in the world; they even have comics book prices on pricecharting.com. Find your special card and then find it on eBay for a lower price. Type it in the search bar, go down to "buy it now" then click "lowest price + shipping" and you'll see the lowest price at the time for that Pokémon card from individual sellers. These deals are everywhere and you can't really miss them.
Sell the pokemon cards that are in rotation, you don't have to, just know which cards are, you could make more money.
Playing the long game with these cards. Most cards will sky rocket over time but 90 percent of the time the curve will even out unless something big happens in the industry, like some new Pokémon burger shop. Pokemon cards move like stock, just a little bit of news will make almost every card go up, that's how addicting they are.
Pikachu, all starter Pokémon, like charmander, squirtle and bulbasuar and every other series. All full arts, they go perfect inside a case. i.e. graded 10 or whatever you can get. Promo cards also look good in a slab. Anything with the pokemon pack logo like from prize packs, mcdonalds, or championship packs. Mew, Mewtwo, Geninja (gold stars). Not just rare Pokemon but those everybody likes, if you haven't seen the show you might want to watch it... it's a very long series.