From the Amazon website? No. From the products they're selling? Yes, absolutely, and lots of them do, I get one of those business cards with "Find us on Amazon/Ebay/Shopify/whatever" in the box with almost every purchase.
Same with apps. I obviously don't expect them to link to items from other stores from their App Store description pages. But from their application? Yes, I totally expect that.
That's how marketplaces everywhere work, including IRL. Go to any farmer's market and most sellers will give you a business card with their website or phone number so you can also order from them directly, or from their Amazon/Shopify/whatever page.
Edit: not to mention that this is 2025, the distinction between "within the app" and "via your website" is pretty meaningless in a bunch of cases.