@Jeruba: Not as likely as you might think. Some people just want to get rid of their collections and will sell them cheap or just give them away.
I would suggest going to a local card shop and asking to see their buylist. This is a list of cards that they buy and how much they buy them for. Card shops tend to buy cards for about half of what they sell them for so if you have a card that they’re selling and they sell it for ten, you’ll most like get four or five for it.
Also, go through the rares. Nobody cares about the commons and uncommons (well, a very few are worth something but those are kinda hard to find) so pick out the rares and set them aside and try to sell them first. Keep the cards in as best condition as you can. A lot of places will not buy cards from you if they’re not in near-perfect condition. This is a good website to check the prices of cards on. This site will help you figure out what the rarity of the cards are. If the cards are from before a certain set (I can’t remember which) they all have black symbols, which nowadays mean common. Breakdown of rarities:
Black: Common
Silver: Uncommon
Gold: Rare
Orange: Mythic Rare
If you’re not sure how to check for rarity, look for the symbol on the right hand side of the card between the picture and the text box.
If you have any other questions, PM me and maybe I can answer them. :)