I work with a guy who is into baseball card collecting, and was asking him how to go about figuring out values (I have a bunch of old cards that I collected when I was a kid, and am thinking about getting rid of). He said that checking out Ebay is your best bet to get a true market value, as the prices for sold cards on Ebay are what the market is bearing, more or less. If you have a truly rare card, or one in mint condition (or both) you may want to get it valued, but those services do cost some money.
I don't actively collect baseball cards anymore, though. I stopped probably in high school when I got my first car. I used to like older cards from the fifties and sixties, and used to buy cards for legends that I liked, but didn't worry too much about condition or future value. Consequently I have lots of cool cards that are in less than pristine condition. I've considered getting an Ebay account to flog them, but don't want the hassle of dealing with "collectors" who might decide the card's not as described and will want money back and whatnot. I prefer dealing with people face to face.
I considered selling some of them off years ago, asking about it at a card and collectible show at a mall. I was met with a bunch of old guys acting like jerks towards the punters. It led me to believe that baseball card collecting has been ruined by collectors.
-William