Your Best bet is usually Craigslist.

I like to use e-Bay or half.com. It sounds like a hassle, but it really isn't. You can set your price and cut out the middle man so you get more for your books while the buyer pays less than they would at their bookstore. Plus, you can sell editions that bookstores around campus won't buy back. It also doesn't take much time because you can tell e-Bay what book you have and it will fill in the description for you, as well as print a shipping label when the book sells. You can arrange for at-home pick-up of packages and save yourself a trip to the Post Office, or there's a Post Office in the Colorado Bookstore up on the Hill. 

I'm going to second half.com.  You can set your own price, and it's almost always a better price than what buybacks will give you.  Yes, you have to be responsible and get the book in the mail in a reasonable amount of time, but if you're looking for the best price, nowhere in town is going to beat the internet.

Craigslist or eBay

Amazon, definitely worth the extra time to actually make some money back. everything else is a scam.

you can try facebook marketplace, or amazon.com. also try posting your books on craigslist