It's very unlikely you have a copy of Action Comics #1 (featuring the first appearance of Superman) or Amazing Fantasy #15 (Spider-Man's debut) — but even more recent comics can have high collector's value, if they're in good condition. Keep an especially sharp eye out for comics featuring the first appearances of popular characters. For example: Amazing Spider-Man #129, from 1974, features the first appearance of The Punisher. A good-condition copy of that issue recently sold for more than $4,000 on eBay.
Quick Cash Tip: If you've already tried selling your comics on eBay with no luck, check out Sell My Comic Books. As the name suggests, it's a website dedicated to buying old comic books. Getting an appraisal on your collection is free and simple.