Search any supplement name and the first page of results will contain the brand's own checkout, three marketplace listings, a dozen affiliate pages and at least one thing that is a straightforward scam. Telling them apart takes about ninety seconds if you know what you are looking at, and costs you the refund window if you do not.
Why it matters more than it sounds
The money-back guarantee attached to most direct-sold supplements is a contract between you and the merchant. Buy the same bottles from a marketplace reseller and that contract does not exist — you have the marketplace's returns policy instead, which is usually shorter, usually excludes opened items, and has nothing to do with the sixty days advertised on the brand's page.
Storage is the second problem. A bottle that spent four months in an unconditioned warehouse is not the same bottle, and nobody in that chain is accountable for it. The third is simple authenticity: repackaged and counterfeit supplements are a real category, and the resale channel is where they turn up.
The four checks
Where does the money actually go? On the checkout page, look at what the payment form belongs to. A direct merchant checkout will be the brand's own processor. If you are handing card details to a marketplace account, you are buying from a reseller regardless of what the listing title says.
Does the guarantee match? If the sales page says sixty days and the checkout says thirty, or says nothing, the guarantee you read about does not apply to the purchase you are making. This mismatch is the single most reliable tell.
Is there a named support desk? A direct merchant publishes an email address and a company name that you can find elsewhere. A reseller usually publishes a storefront handle and a contact form.
Is the price too good? Direct sellers set the floor price themselves. A listing well under the brand's own lowest per-bottle price is not a bargain someone forgot to close — it is a signal that you are not buying what you think you are buying.
What an affiliate page is, and why it is not the same thing
A page like this one is a third category and it is worth understanding the difference. An affiliate page does not sell you anything and does not hold stock; it writes about the product and links to the merchant's checkout, earning a commission if you buy. You still end up on the official checkout, so the guarantee still applies.
That arrangement is fine as long as it is disclosed. What is not fine, and what you should walk away from, is an affiliate page dressed up as the brand — the brand's logo as the site identity, first-person language as though they made the product, a domain chosen to be mistaken for the official one. If a page will not tell you plainly whether it is the manufacturer, assume it is not, and assume the reason it is being coy is not in your favour.
A ninety-second routine
Open the page. Find the sentence that says who runs it. Click through to the checkout and read the merchant name on the payment form. Compare the guarantee on the checkout with the guarantee on the page. If all three line up, you are buying direct and covered; if any one of them does not, close the tab and start again from the brand's own site.