It may not be everyone's ideal purchase, but if you have a comfy, clean mattress you no longer need, likely someone will haul it away for a reasonable price. Approach the mattress-seeking masses from one or two angles to increase the chance of a sale -- without breaking the law.
That's right; it's for the sanitizing process.
Write your ad, including pertinent information, such as size -- twin, queen, king -- condition, age, thickness, firmness, whether the mattress comes with a box spring, and its make. Some mattress makes and models are costlier, longer lasting and better all-around quality than others, so do your homework. If you can't remember how much you paid originally, find out what similar mattresses are selling for now, so that you can price yours accordingly. If it was used on a seldom-slept-on spare bed, advertise it; if it was always protected by a mattress pad, note that too. Let potential buyers know what type it is -- memory foam, innerspring or pillow top, for example. Depending on condition, list it for no more than half the price of new to generate interest -- and be ready to dicker.
Place your ad wherever it seems practical; if you live in a small town, for instance, you can simply print a few ads for local bulletin boards, post it for sale on your social media feed, run it in a community mini-magazine, and even mention your ware to trusted resident gossips, such as your butcher, baker or specialty coffee shop. In a city, or if you live near a large center, advertise the mattress on auction sites or in local newspapers.
To increase the chance of a sale, include a picture of the mattress, zooming in on the fabric and any tags -- such as a brand or cleaning company's label -- and note whether you can or cannot deliver it. If you can offer delivery, list a nominal fee to cover your fuel and time.