Hi maggirus,
In some cases yes. Although a used mattress that has been returned and disinfected should have a tag on the mattress to identify it as being used (this varies by state) and would normally be sold at a deep discount at a liquidation store … in some cases some of the more “shady” stores will sell them as new or as “floor models”.
There is more about what happens to mattresses that are returned or exchanged in post #2 here and there is more about return and exchange policies in general in post #3 here and the other posts it links to.
Whether a return or exchange policy has value to you would depend on your confidence that the mattress you are purchasing is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and on how carefully and accurately you tested it before a purchase. The cost of all return and exchange policies are built into a mattress and in stores that have return or exchange policies the customers who don’t return or exchange a mattress pay for the ones that do. All of this is just one of the tradeoffs that are part of each person’s personal value equation and the parts of a mattress purchase that may be more important to some and less important to others.
The best place to start your mattress research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choices, identify all the tradeoffs that are most important to you, and know how and why to avoid the worst ones.
The better options and possibilities I’m aware of in the San Jose area are listed in post #2 here.
Phoenix