I sell books on Amazon that I don't want anymore. You set the price and wait for somebody to pay it. If you set the lowest price for a popular book it will usually sell within a week or two. Less popular books may take longer.
Financially, the way it works out is that Amazon charges the buyer the price you set plus $3.99 for shipping. Out of that sum, they take a fee of $2.34 + 15% of the item price (the fee may be different for non-book items), and give the rest to you. You can pay the shipping cost right through their website out of what's left over. USPS media mail shipping should cost less than $3.99 for all but the heaviest textbooks. You can then pack the item, weigh it, print a shipping label, and stick the item in the mail.
Amazon does have a trade-in program for many of the more popular items, but I find that the amount you get is almost always less than you would get if you sell it yourself through their site. Once you get the hang of selling, it's not even that much less convenient than a trade-in. Plus the trade-ins are paid in Amazon gift card credit rather than actual money.