I consider "old comics" golden, silver, and bronze age comics. Books that are from the 90's are NOT old. Yeah, I know they're 20 years old, but that's not old at all in terms of comic collecting. Most 90s comics are a complete waste of time.
That's the decade when the brilliant people in the comic book industry decided to overprint comics more than the demand for them. So now, most 90s comics are completely worthless. There are exceptions of course, but on the whole, I would not invest in them.
If you have a lot of old comics (Golden & Silver Age) there are quite a few places to sell them. A lot of dealers and investors are specifically looking for books in these eras. So here's a list of places to sell them:
1. Comic Link is an auction style site that's only geared towards comics. You can sell by auction, consignment, or sell direct to them if you want to get rid of them fast. They mostly don't bother with most modern age comic books, especially those from the 90s era unless it's a high demand key issue or low print variant cover.
2. Local Comic Dealer - This is surely an option, but I don't recommend selling direct to them. Well, if you don't care about getting ripped off and need money fast then this is the easiest route to go. However, what I do is consign my books.
They take a 30% cut, which is a lot, but it's an avenue I use a lot.
3. Craigslist - You sell can almost anything on Craigslist. It's a great site to put up classified ads in your area, and a lot of people visit Craigslist. List your old comics for sale there, and you may grab a buyer. I've sold a lot of stuff there.
4. Garage Sale - Have a garage sale and advertise it in Craigslist. This is a great route if you have 90s comics and just want to get rid of them. Silver Age or Golden Age books? You wont get too many buyers. After all those who frequent garage sales are looking for that valuable item they can get super cheap.
5. ebay - This is my second to last choice in selling old comic books. For most books - even silver age or golden age - are sold on ebay well under guide values. Ebay is a terrible place to sell your comics, but a great place to buy them.
6. Comic Book Conventions - If you have a major comic book convention that's held annually in your area, you can peddle your old comics to the dealers selling there. Be prepared to haggle if you got really great silver age or golden age issues.
Remember, if you have a lot of 90s books, pick out the ones that are valuable. The remaining are junk comics that everyone else is trying to get rid of too. You'd be better off donating them to a library or donation center. Read my post on Modern age comic investing - A good or bad choice?
These are just some ideas to help you answer that question - "Where can I sell old comic books?" If you've got more ideas, be sure to comment below. I'd love to hear more ideas.