Doing a simple google search will return a plethora of online vendors offering to buy your textbooks. Individuals also buy and sell textbooks as a side hustle. But who is going to consistently pay the most money and buy the largest variety of books? Some vendors are super picky about which titles they are willing to buy, and they only buy the ones that they know they can turn around and sell for a profit. You're going to want to stick to a few top companies most of the time because they are going to consistently pay you top dollar. I'll list a few of them below.
Amazon.com - Yes, Amazon will buy your books, and they have, obviously, the largest online retail presence in the world and the most power to resell. They will usually give you a good price.
TextbookRecycling - A relatively small, unknown company, TextbookRecycling consistently bids near the top for most books. This company is usually willing to pay you near, equal, or more than many of its competitors. They have a good buyback program and are definitely worth your while. Like most vendors, TextbookRecycling does have a minimum buyback of $50 for which guarantees you free shipping.
ValoreBooks - This company has a nationwide presence, and at some universities, they handle all the book buying/selling officially - some schools aren't big enough to have a full featured on-campus bookstore. ValoreBooks has a huge customer base, and they pay good rates for used textbooks.
Chegg - A publicly traded company, Chegg is huge, and it has tried to position itself as the default online solution for all textbook needs. They focus a lot on renting books, which is a highly profitable business. However, you can sell them your used textbooks, but I've found Chegg offers are usually not quite as high as some of the other vendors. But you never know until you check.