There's only one official way to buy the Steam Deck in western markets: the Steam Store. Granted that Asia is served by Valve's partner Komodo. But that's neither here nor there.
I was browsing YouTube this morning and found that there was an ad for the 64 GB Steam Deck on Wal-mart's website.
At first I was skeptical that this was truly walmart.com, but in fact, this is the official Wal-mart website with a listing for the 64GB Deck. And it's not just Wal-mart. Amazon also has Steam Decks on offer.
How is this possible? It's simple: it's a third-party reseller. They're wholly unathorized and there are no guarantees here of this device being new... let alone worth the hundred-dollar mark-up.
These listings also come with no support or warranty, so "buyer beware!"
It may go without saying, but I'd avoid listings like this. Especially any listing that has a notable markup, offers pre-loaded games, or has higher-tier storage than what's officially offered.