So what's the percentage of overlap items are there? The vast majority of Costco's sale are bulk items hence the name Costco Wholesale.
Lets revisit the list I already posted:
appliances,
furniture,
clothing,
toys,
books,
computers,
other electronics,
jewelry, food,
alcohol, soda, water,
jewelry, or
luggage
11/14 types of items are sold singular quantities, not bulk. That means only 22% of the types of things sold at Costco are potentially bulk. You have a really funny definition of "vast majority".
Besides that, you claimed that Costco and Walmart don't compete with each other. It's an idiotic thing to say. If I want to buy food, the fact that Costco food is sold in larger bulk packages is hardly a deciding factor. I can compare the cost of buying equal quantities from each store and make a decision. They clearly compete! There may be difference in business practices behind the curtains, but the end result for a customer is very comparable!
If a box of cereal at Walmart is $3 and a bulk double box of cereal is $5.50 at Costco, I can compare and find that Costco is cheaper by $.25 per box. OMG, I did the impossible and compared a bulk item to a non-bulk item!
Here's an article you need to read so you can understand why Costco can afford to pay their workers more than Walmart.
I'm aware, and it's an awesome thing that Costco has such an efficient system going on. What exactly is the point you are trying to make? Costco is run better than Walmart? I wouldn't disagree. Would Walmart crash and burn if it tried to pay it's employees the same as Costco? I can only dream.