What makes an old comic book valuable?

The biggest factors are the title, the issue number and condition. First appearances of major characters, first issues of important series and key storylines draw the strongest demand. Comics from the Golden Age of the late 1930s and 1940s and the Silver Age of the late 1950s and 1960s are scarcer because many were read, traded and thrown away.

Condition is graded on a detailed scale, and small flaws matter: spine wear, creases, tears, missing pieces, water damage, writing and yellowed or brittle pages all lower value. Restoration, even well done, reduces value when it is discovered, which is why professionally graded and sealed copies note any restoration.

Most comics from the 1990s were printed in very large numbers and sell for modest amounts, though there are exceptions. Look up sold prices for your exact title, issue and variant in similar condition. For important older books, a comic dealer or auction company that handles comics can tell you whether professional grading makes sense.