A few different ways. They buy large quantity of factory overstocks. The companies sell to them at a loss just to get inventory off of their shelves. They can then sell them cheaper because they bought them cheaper. It's cheaper for big companies to take a loss on an entire lot of items than to carry them over in inventory and try to move them.

They also likely get products similar to a store here where I live. They get prducts out of insurance settlements that were in semi or train accidents. Even if the products aren't damaged in an accident they still can't be sold at a retail store. The insurance companies pay out and then sell to these companies buy them for pennies on the dollar. That is why they carry such a variety of products.