There are a few key criteria that make a comic book rare.
The first and most useful in determining if that comic book in your storage closet is rare and potentially worth a lot is whether or not that comic book was a limited edition run or print. Many comic books were printed in limited quantities and some (like collectible cards) will even have a number (ie.- 5 out of 500) somewhere in them. The less that were printed then means there are even less in perfect condition today. Of course if you have a pristine copy of a comic that had a print run of 100,000 copies, its worth is far less then owning a pristine copy of a comic where only a few hundred were made. Although you’d still have a slightly rare good condition comic if out of those 100,000 copies only 5 or 10 were in perfect condition.
Secondly current market demand dictates the rarity in terms of dollar value of a comic. Sure if you own some comic named “Piggy Oink Oink’s Adventures” and you own 1 of 2 remaining copies of a 10,000 print run, the comic is ‘rare’. But unless there is some market demand or underground cult fascination with said comic, its value will not be as great as owning 1 of 100 remaining copies of say the first Superman comic ever printed.
Condition of the comic book is also crucial. Collectors will pay top dollar for a rare comic, but because they are investing potentially a large sum of money they want the copy they buy to have something unique about it; or at least be the best of the best. That said any folds, creases, bends, stains, or marks in the comic can severely reduce its monetary value. If a collector had a choice between your comic which was the 2nd of 500 printed but is damaged and the 10th of 500 in perfect condition; they would choose the latter since in the long run that one will remain in better condition then your copy.
Also a unique comic book will often become rare. Such as my fake ‘Piggy Oink Oink’ example, if that was in fact a rare comic and only a few remained, it may become valuable if the market dictates such oddity comics hold value. Comic books which contain misprints may become rare as well, especially if the error was caught early in printing. And of course comic books which contain an artist’s autograph become very valuable if there is a market demand for that artist’s work.
So many conditions contribute to making a comic book rare and valuable; go and look through your collection and see what you uncover. Many times you can find ‘rare’ comics on Ebay; if you have the same one you can roughly gage the rarity and financial worth of the one you have stored away. Just remember to keep all your comics in excellent condition and stored in a cool, dry location. If you have comics sealed in original packaging, don’t open them. You never know which comics will become the next big collector item.