What's the best site to sell used textbooks online?

Amazon

There’s eBay (but that’s too big to really narrow down things)
I think Barnes & Noble’s will let you sell used textbooks too. I think that would be best. Maybe.

My college had a internal textbook selling site where you sold it directly back to the students. It started off as the software engineers class project or similar. Maybe you should ask around and see if your school has that too. I got a lot of money out of it and never had to pay more than $300 per semester using it.

I kept all of mine for future reference.

As a matter of fact I probably would.

Mostly Amazon!