Commonly used websites like Craigslist and Reddit are great ways to sell locally. Things in relatively good condition can also be sold through Amazon, and once you’ve listed something to sell, you can send it to Amazon’s warehouse so that you don’t have to keep holding on to it. I recommend Amazon for selling books that aren’t textbooks, as long as they’re in relatively good condition. You can also find local and student Facebook groups for buying, selling, swapping, and giveaways. There’s a great local page dedicated to buying, selling, and swapping used cars and trucks, and a UIUC student page for furniture.
As for textbooks, Bookbyte, Valore, and Chegg are just a few of the websites that offer buybacks. Your ability to sell to these websites is not affected by whether you’ve ever bought books from them or not, and usually they will also pay for shipping. You can also list books to sell on Illini Book Exchange, which is a website specifically for UIUC students.
For people who like exploring, there are usually plenty of yard or estate sales to be found around Champaign-Urbana and surrounding neighborhoods. Additionally, Jane Addams Book Shop in Champaign and Secondhand Rose in Urbana have the feel of a garage sale and are each full to the gills, Jane Addams exclusively with books and Secondhand Rose with everything else. Be aware that buying from these places is more of a novelty and not as cost-effective.
For additional options, University Ave has two pawn shops, Pawn King and Leonard’s Pawn Shop.
Lastly, something you might not know is that the university sets up donation boxes in the dorm lobbies at the end of each school year. Oftentimes you can find brand new or only slightly used stuff in these boxes -- in addition to providing an outlet for getting rid of your old stuff, they can also be a good place for stocking up for next semester.