There are a lot of good reasons to buy used furniture.
Older wood pieces that are still in good shape are often better quality and almost always far less expensive than more recently made similar items.
But I would never buy a used mattress.
I would also never buy used upholstered furniture, unless I was planning to have it completely reupholstered.
There are too many hidden problems that could be lurking in mattresses and upholstered items including (but not limited to) bedbugs, insect or rodent infestations and mold.
Used sofas made within the past ten years may look OK, but there is a high probability that the cushions will be worn out to the point of discomfort.