Many people discover that they have comic books from years ago and they suddenly think they may have hit the jackpot. A few things to keep in mind.
Just because a comic book is old, doesn't mean it has value.
Collectors look for many other keys to value a comic book, such as:
1. First Appearances: Was it a first appearance of a certain character? That doesn't mean that the comic has a #1 on it, it means did they introduce a new character that would eventually become popular. A prime example is the issue of The Incredible Hulk #181 in 1976. What would seem to be just another monthly issue of this book, except it introduced a new enemy to the Hulk, The Wolverine. Well, we all know that with the popularity of Marvel movies decades later, this unassuming book became a holy grail of comic book collecting valuing in the thousands of dollars. I once sold a mutilated horrible copy in a 1 out of 10 scale condition for $800
2. Story lines - Was this a story line that captivated readers for decades?
3. The Artists - Certain artists just redefine the industry and any artwork that they were involved with increases the value. A good example is Neal Adams. In many cases they would contract him just to do the cover! not even the inside art just the cover, and those issues have a higher value.
4. Condition - This is a world that can take you a while to learn. Collectors are basically OCD about the condition of a book. A small nick of 1/8 of an inch, which can happen just by picking up the book and turning a few pages, will start the decrease value process. Folds, tears, stains, fades, etc, etc, each take their toll on the value of a comic. Grading is also subjective, so opinions will vary. That is why there is are grading companies, such as CGC, that will professionally grade your book and encapsulate it so it is sealed forever with that grade. Prices for grading start roughly at $25 a book, plus shipping to and back. It can get costly.
5. Trends and Hype - If a new TV series comes out about a character, first appearances of those books will temporarily increase the value of the book. Another example of mine: I had a copy of Avengers #57 which is the first appearance of The Vision. I had it professionally graded at a 7.0 grading by the CGC. For years this book values in the $200 range. Suddenly the TV series WandaVision came out and the book gained popularity. I parted with it for $600. In 6 months it will go back down again if not sooner.
6. Other factors - Who wrote the story? Was it a short publication run making it rare? etc etc. I am probably missing a lot of other things that collectors look for and I will ad them to this article as time goes by.
I am not looking to minimize the value of anyones collection, as I do buy collections myself, but just because you have books from the 80s and 90s (which may be older than you are) doesn't mean they have value. In fact, the 80s and 90s was the start of the biggest decline in comic book history value, and forget the 2000's. Ninety percent of the books of these eras have very little value, in many cases way, way, way below the cover price! That's right you paid $2.00 - $3.00 per book for 200 books, so you spent $500. Well, they will probably only value at about $100 if you are lucky, regardless of condition. The other 10-15% of "Key" issues will have values from $5 - 500 in some cases more. So unless you are willing do go to Comic Book University for a year (that school doesn't exist, just made it up but you know what I mean), then just part with whatever the dealer market offers you or give it to someone who will appreciate the gesture. Comic book collecting is fun, comic book selling is another story.