I work in the IT department for a fairly large payment service provider. I can tell you now that you seem to be vastly underestimating both the financial aspect of this as well as several legal aspects.
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Federation would almost certainly have to be opt-in rather than opt-out. I don’t think you’re going to pass KYC checks for any PSP if it’s opt-out, the risk of someone (ever so briefly) selling illegal goods through your website is too great otherwise. Stripe would just shut down your account (if they even let you open it), PayPal probably won’t let you open it at all.
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Selling goods from other sites through your own, makes you liable for any returns, warranty claims etc… Simply “passing these on” isn’t going to cut it. If the other site disagrees with the customer claim, you are on the hook for it, because it was sold through your website.
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The financial logistics aspect here is really complex. If you’re going to process payments on behalf of another site, you have to deal with reconciliation. After reconciliation you have to the send the money to the other shop, incurring additional (sometimes surprisingly sizeable) fees. And coming from someone who deals with (automated) reconciliation on a daily basis, every payment method does it differently and they all find extremely creative ways to mess up your systems. And that includes unannounced changes, mistakes, random unexplained fees, failure to deliver settlement files, etc…
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How do you deal with the risk of scam instances? E.g. instance A tells instance B that a product was sold and the payment was processed. B sends it out, but it turns out the customer was the owner of A, and there was no payment at all. B just lost a product with very little chance of getting it back.
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Then there’s practical aspects. How do you deduplicate products in search? Or will you have dozens of listings for the exact same product?
The only remotely viable way I see this working is if only search is actually federated. Once you are on a product page, you can only pay using the payment page of the instance that has the product. You won’t be able to pay for products of multiple instances at once, and you might lose some unified styling. But at least that approach has a chance of passing KYC and deals with all the legal issues regarding returns/warranties etc…, and it reduces the scam risk because you’re in charge of your own payments. But at that point, you’ve only federated product search and nothing else, and then as a consumer you might as well just Google it instead.
I appreciate you have experience in running a business, but running a marketplace, especially a very complicated one, is really not like running a usual business.
I think the biggest issue is that if you already need to separate payments, returns, shipping, etc… you’re left with a shop that also advertises products for other shops, possibly competitors. Then the question becomes… why bother federating at all?
I think it’d be better to set up a FOSS shopping platform, eg something that competes with WooCommerce or the likes. That’s significantly easier from a financial and legal perspective, and I think it’s an easier sell to actual merchants (why pay a license for that shit, use this one for freeee). Then once you have that running, you could think about optional federation as an addition to an already well-functioning platform.