Instructions
Contact or visit used bookstores. Ask whether they are currently buying comic books. If they are, tell them you have a box you'd like to sell. They may have stipulations as to how they want the comics presented or delivered.
Sort your comics so that the people at the bookstore evaluating them can sift through them easily. You can sort them in several ways. For example, you could sort them by title and issue number, for example Action Comics Nos. 963 through 1001. You could sort them according to age or publisher. Presenting an ordered box allows the store to deal with you more quickly.
Take the box to the store. Follow any specific delivery instructions you learned earlier. For example, for large lots the store may want you to call ahead so that someone is available to look through your comics.
Retrieve any unwanted comics. The store will likely create a pile of comics it wants to purchase and a pile of comics it doesn't want. It will make you an offer for the comics it wants and may tell you why it ruled out certain comics. Accept or refuse the offer, then place the remaining comics in the box.
Proceed to the next bookstore if you still have comics to sell. Try all the bookstores in your area to sell the largest number of comics. You can then donate, discard or recycle any remaining comics.