Trust is how you know. I trust Target, Best Buy, whomever, to sell me an actual Dyson battery. I’ve never had any problems with any company not taking an online return (except once with Amazon charging me for a replacement for a ~$10 item they said was delivered but wasn’t). Amazon doesn’t really stand out in that regard these days.

That Vietnamese company with the 3x capacity:price ratio is probably selling junk. There are just so many more junk sellers (at least by name—maybe a handful operating all those names). So it boils down to taking a gamble when the odds are against you, getting lucky, and building a relationship or going to a retailer you trust already.

Those I trust that would sell me the battery I need are buying from Dyson and so out of stock. They also charge 2x for a smaller battery. I'm left with janky brands: either from amazon or from janky companies.

I've had great luck in these situations buying from a small company that filtered out which cheap brands were good products. However I already have some other reason to trust those small companies.

I've mostly switched back from shopping online to shopping in local retail. If you're actually stocking shelves, instead of just running a digital flea market, you have to be choosy about what you stock. There's a ton of value in having someone filter out the trash and buy from reliable sources. I'm happy to pay more for that service, and usually I can get the item same-day since I'm already there in the store. For example you may have a local vacuum/appliance shop who has a direct relationship with Dyson and so can buy genuine parts on your behalf, or even have them in stock wholesale.

There is a lot of truth to that, but it only applies where a local retailer stocks what you are interested in. There are a lot of parts that can't be got locally. There are a lot of hobbies that can't support a local store.