Hi LJGMDAD,
It’s actually not illegal to sell used mattresses … just to sell them without the appropriate law tag for the state you live in (see post #2 here and post #2 here and the posts they link to).
Different DIY sellers would handle this in different ways (see post #2 here for some of the possibilities) but there aren’t as many exchanged or returned layers as most people would expect since most people are happy with their original choice. No reliable seller with a good reputation to maintain would take the risk of selling a used layer without letting a customer know that they were receiving a used or exchanged layer … the risks would be too high. Having said that … there are also many customers that would gladly purchase a used layer for a discount if they happened to be available.
A reliable seller will give you truthful information about exchanged or returned layers.
In the specific case of Mattresses 24/7 they used to sell layers that were comfort exchanges years ago (and disclosed it in their descriptions at the time) but they no longer do and all the layers they sell now are new.
Phoenix