There are several reputable trading card appraisal companies, including: PSA/DNA, Beckett, GAI, among others. Some have more clout than others, depending on area of expertise (i.e., James Spence, autographed items).
Condition is an important factor, so it is best to take it to a sportscard shop for an appraisal, or check the current Beckett Magazine for an approximate price.
Sports card appraisals can be found at sports card stores . Always get a few opinons. There is a great website called "Baseball Card Road Show. Also, check the libray.
Well you can go to a collector to see what your cards are worth. Another thing you can do is just list them on ebay or maybe check what the rates are for your baseball cards. Some people are willing to pay 100's for a certain baseball card over others.
The best and most reliable appraisals can be obtained at national card shows. The major companies that value cards do on site appraisals at many of those shows.
