After you know the expansion it is a good idea to figure out if the card you have is a misprint or if it has some sort of graphical errors. Pokemon cards which have errors are usually more rare then normal cards in most cases but this is not always the case. Some cards are actually more rare when they are not error cards like in the example of some earlier Pokemon cards which had the wrong artist for card design and very few of the ones with the correct artist where actually printed making the originals more rare then the actual error card.
After you have discovered whether the card has any errors its a good idea to then figure out whether the card is rare or not for the expansion usually cards are marked by rarity. Holographic cards are usually very rare and there are also super rare holographic cards in some expansion as you can see when you check the Pokemon cards list.
Most of the older expansions are no longer printed this makes many of the cards more rare and first edition cards are usually the most earliest ones printed. But not every card will be valuable especially when it comes to websites like eBay where people have to bid on an item. Bidding on items can actually make the prices very low compared to what you would buy them and spend from a retail Pokemon card store. So the best way to understand the value of the cards is first by checking what you would pay for the card in a retail store. Then once you have that price then figure out the condition of the card.
The better condition of the Pokemon card the better the value will be. If the card is really damaged the value of the card will be decreased by a lot. Most collectors will be looking to buy Mint condition cards. But if the condition is bad it does not mean it won't sell many people that play the trading card game use the more non mint version of cards since they are not collectors cards and get beat up more. After you have the condition then you can compare the different cards on eBay to see if any are being sold but you would do better selling through a website like amazon. Of course prices change constantly but I will attempt to provide a price guideline for Pokemon cards.
|Pokemon Diamond & Pearl||Unlimited||$179.99|
|Pokemon EX Crystal Guardians||Unlimited||$349.99|
|Pokemon EX Crystal Guardians Reverse Holofoil||Unlimited||$199.99|
|Pokemon EX Delta Species Reverse Holofoil||Unlimited||$249.99|
|Pokemon EX Deoxys Reverse Holofoil||Unlimited||$249.99|
|Pokemon EX Dragon||Unlimited||$349.99|
|Pokemon EX Emerald||Unlimited||$299.99|
|Pokemon EX Fire Red & Leaf Green||Unlimited||$499.99|
|EX Hidden Legends||Unlimited||$349.99|
|Pokemon EX Holon Phantoms||Unlimited||$329.99|
|EX Legend Maker||Unlimited||$329.99|
|Pokemon Cards EX Team Magma vs Team Aqua||Unlimited||$379.99|
|Pokemon Cards EX Sandstorm||Unlimited||$279.99|
|Pokemon EX Team Rocket Returns||Unlimited||$399.99|
|EX Unseen Forces Unowns||Unlimited||$129.99|
|Pokemon EX Unseen Forces||Unlimited||$369.99|
|Pokemon Legendary Collection||Unlimited||$299.99|
|English Pokemon E-Card Skyridge||Unlimited||$649.99|
|Pokemon Cards Platinum Arceus||Unlimited||$249.99|
The list above is only something you'll want to use as a bases when buying and selling keep in mind condition play a huge factor these are also the unlimited edition's. The first edition Pokemon cards sell for a lot more and more money can be made per single card. The sets above's price are mostly determined by the value of the holographics contained because those cards are the really rare peices while most common cards are much lower priced.
There are rare pokemon cards that are much more valuable then the pokemon card values above. The prices surprisingly have been slowly increasing over the years especially as more collectors jump to gather all of the sets from the begining to the most current set.