Literally, the only things you negotiate in the US are cars and houses. Not groceries nor clothes nor millions of other things. Perhaps a few minor things here or there.. but we pretty much just stick to the basics.
Mattresses are negotiable. I offered to pay cash and they took like 40% off the price. I was kind of dumbfounded.
Did they put it through the books. They may be able to sign it off as damaged goods and pocket the cash hiding the sale from revenue and upstream managers.
First time I bought a mattress I wasn't aware of this. Second time, I got the bedframe, box spring, and mattress protector included, and still managed to knock ~ %20 off the price. I try to negotiate everything now, and have had some measure of success. I will admit that negotiating is mostly, to me, a fun game, rather than a way to save money.
That depends on how broadly you define negotiating. For example, price matching and acceptance of competitor coupons are both forms of negotiating. But you're right, even for retailers that offer price matching or competitor coupons, the vast majority of purchases don't involve any form of negotiating.