WM buys a lot from local, just ask a store manager.
And do you know how they buy local?
Let's say you are a farmer and you grow tomatoes. Growing your tomatoes costs 5/kg so you sell them to the store for 7$/kg and the store sells them for 9$/kg.
Walmart comes along and says "I will buy some tomatoes for 7$/kg and sell for 9$" so you say ok!
Then 6 months later they says, we want to buy all your tomatoes, but we'll only pay 6$/kg, but we'll sell for 8$ so we will sell more. You say ok because you make less money, but at least you are guaranteed to sell everything you make so it's all good in the end.
The problem is that you had to drop everyone else who was buying from you so you only have 1 buyer. And all other buyers are pissed because they are losing sales because Walmart is undercutting them.
So 6 months go by and Walmart says, we're only ganna give you 5$/kg from now on. But they still sell for 8$.
Now at this point they have you by the balls. You can't say no because they will stop buying and no one else wants to come back to you. So you say yes and you pay your employees less, you cut your own income and you keep going.
6 months go by and Walmart says now it's 4$/kg and still sell for 8$ and so on and so on until you go out of business and they just move onto the next supplier.
Walmart is a cancer that destroys economies everywhere it goes.