Uhhh LTT has lots of experience. They're a team of tech professionals. Linus in specific definitely has experience building with Chinese parts, they have countless videos on that.
AMD does not make PSUs. Additionally an 850W silver PSU should not cost that much. Gold is a higher rating and costs less than that new.
Chinese resell sites are not the place to look for un-opened products. They'll sell you just the box for an opened product and call it new. And then ship it in 5 months. Would not recommend. EBay and Craigslist are where you would find local listings for used products. Additionally microcenter and the manufactures might have listings for used products, but most do not and resell to other third parts like amazon or newegg as "Manufacturer refurbished".
3090s just got released yesterday, the ti won't be out for most likely another year, but yeah? I guess don't buy a 3090ti when you're looking for a powersupply. Good advice