Haha, there is definitely a nice, big selection of mattresses! They seem to be popular on the site.
You're not the only one who's noticed. OfferUp actually partnered with Beds for Kids last year and we were able to help donate 300+ beds for kids in need. We're doing it again this year and our goal is even bigger - we hope to donate 600+ this time! If you're interested, you can read about it here.