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.