They're not "in Walmart", they sell through Walmart's online marketplace. Just like Amazon, almost anyone can do that. You just pay a cut of sales to use their system and fulfill it yourself, or through a 3rd party. Mom and pops, and penny scams, do this all the time. Your grandma could sell quilts via Walmart.com. And so can Grisaffi.
Walmart has almost no participation, except to extract a fee for using their ecommerce engine.
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More info directly from Walmart:
Walmart Marketplace Buying Options
Here's what you need to know about buying from our approved Marketplace Retailers:
Marketplace items are not returnable to Walmart.com or Walmart stores
You can buy Walmart.com and Marketplace items in the same order
It's safe - we don't share your credit card information with Marketplace Retailers
Marketplace Retailers ship the items you buy from them
Tracking details (if available) can be found in your Walmart account
Marketplace Retailers provide customer service for the items you buy from them
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