It has happened to me a few times that I order from Walmart, see a shipment notification, that shipment "arrives" but definitely did not. However that day or the next I receive an Amazon package which I did not order containing my product.

So no, ordering from Walmart does not guarantee anything is going through their supply chain.

Was that from Walmart the retailer or someone else selling on Walmart's marketplace? I would be very surprised if Walmart itself resold you something from Amazon, but that does happen with third party retailers on their marketplace.

This happens in more than just Walmart - there are arbitrage experts that list product on eBay, Walmart, etc and just order it for you from Amazon.

I’m not exactly sure how they make money as the prices are often nearly identical.

I use to do this successfully for many years. I could sell for a few cents less than Amazon. Due to buying gift cards at Kroger with a 5% Cashback card. In addition I would also get 1000 fuel points per $250 that I could sell for $20 easily. So each $1000 in sales would net minimum $130 margin. Accounting for selling fees of around 10%, I could clear around $30 per $1000 even at breakeven or below Amazon prices. I would also be able to sell many items for more than the Amazon cost.

It worked well for several years with millions in sales, all automated. It became untenable though as Cryptocurrency facilitated people buying Amazon gift cards with stolen credit cards and cashing them out at 30-50% of face value.

So the market became saturated with scammers selling considerably cheaper than Amazon. For a time maybe 6-4 years ago you could get pretty much anything from Amazon on eBay for around 20% less and shipped direct from Amazon via prime.

Amazon eventually shut it down though and if you meet some very strict gift card threshold they will just steal your balance. People still do it, but large scale automation isn't really worth it any longer.

I’ve often thought it was something like that, milking various rewards options.

There isn’t a clearly identifiable difference anymore.

There is. The retailer is listed.

Subtly. Most people aren’t going to notice.

Not anymore, I got a counterfeit control pad from walmart.com. Thankfully I was able to return it at the store.