What do manufactures do with return mattresses? Concerned that I might get one someone returned.
Hi ptw1skers1,
Welcome to the Mattress Forum! 
There really are quite a few avenues that used mattresses can travel.
If an item is a warranty exchange and returned to a brick and mortar store, it is usually returned to the manufacturer for a credit. If it is a comfort exchange and returned to a brick and mortar store, it can be donated to charity, sold in a “clearance center”, used as a floor model, sold online, sold to liquidators, or worst of all – sold again to someone and represented as a new mattress.
When the major manufacturers receive their warranty exchanged mattresses from the retailers, they generally arrange them in semi-truck lots and sell them to businesses who operate as “scratch and dent” or “closeout centres” that retail these items to the public without a warranty.
Many online-only retailers have taken the approach to have any comfort exchange or warranty-return items to be donated locally, whenever possible, to a charitable group, by either the customer or they arrange to have the item donated by a company that they contract. If that’s not possible, they often arrange to have a local moving company pick up the item and dispose of it at a recycling facility. This ends up being more cost effective. Some companies, especially with latex and component layer systems, will have the layers returned, in which case they are usually donated, disposed of, sold off to other industries (such as carpet underlay) or sometimes put together as used clearance items for sale locally. Having something like this sold as new online is not something normally found.
There is more detail about this in this thread.
Phoenix