You can do it online, but that's a lot of time and effort on your part for a whole lot of not much money.
The easiest way would be to take it to any scrapyard / auto recycler in your area. They buy used parts just as often as they buy junked vehicles, because used parts is how they make their money. I know your radiator is pretty new, but pretty new means it's used. You won't get full retail value of course, but you'll unload it quickly and there's no guarantee you'll get more for it by selling it online.
You can sell your radiator on the eBay website.
Ebay. Include a photo.
Craigslist. Make sure to get cash.