We always recommend you look up card values before trading, selling, or buying cards. Here are a few places that people commonly agree on for card prices.
General Card Prices
Verity, and price comparison between some other sites.
MTG: Card Prices
MOST USED FOR Magic: The Gathering
Wieb: Card Prices
Sets are along the left side of the screen select language to reveal the list of sets.
UFC: Card Prices CA
The go to for Canadian UFS card Prices
Note: Condition of cards will effect value. Learn More about Condition Here.