Not everything, but most stuff. They sell meat, fruit and veg, stationery, books, clothes, TVs, photo services, opticians, tyres, alcohol, toys, garden stuff, furniture, bedding, towels, cakes, tinned food, frozen stuff, most stuff really. It is definitely worth a look.
I have two shopping lists - a supermarket one and a Costco one. I go about twice a month, sometimes every three weeks.
They are great if you are having a party - they do huge platters of food very reasonably priced.
The meat is amazing quality. The alcohol is not as cheap as offers in supermarkets. The Christmas stuff is fabulous!
You get used to buying in bulk when offers are on. I buy toilet rolls in their hundreds!