Each trader/seller sets their own values based on what they feel is right. There is no hard set value for any card or piece merchandise, and as a result, there are a few different ways to assign value to cards.
Sold eBay listings are one of the best ways to find card value. That is because the sold listings tell you the price that someone else was actually willing to pay for that card. For that reason, sites like Troll and Toad are actually poor indicators of actual card value. For one, T&T is working to make a profit, and because they are a well-known retailer (as opposed to a regular player/collector) they can (and do) charge more for their cards.
Another good site to use for finding card values is TCGPlayer, as they are a marketplace for multiple sellers and they have a Price History indicator that shows how the prices of cards have risen and fallen over time, which will help you get an idea of the values both currently and in the past. Check out their page for
Alakazam-EX as an example.
Another aspect of card valuation that gets overlooked in pricing is individual demand. If a person really wants a particular card or needs it quickly for a tournament, they may be willing to pay/trade more to get it. On the other hand, is a trader has a card that he or she is willing to trade/sell but reluctant, they may value it higher than a given market price and only be willing to trade/sell it if they get a good deal. Similarly, if a trader has cards that he/she doesn't want at all and wants to get rid of quickly, they might be willing to take a lesser trade for them.
Overall, the most important thing is that the trading partners must work together to find a value that is appropriate for everyone involved.
