Go the manufacturer site and see if they will honor 3rd party warranties. Evga would if you could rustle up proofs of purchases while Zotac would only honor warranties if purchases from an authorized reseller.
So you need to know the make and model of the card, who built it.
The warranty terms might also be limited eg. you don't overclock or undervolt the card or remove the heatsink to repaste- so after your warranty has expired it's totally at your own risk 3-5 years later when you need to renew thermal solutions and thermal pads and de-dust things etc. then you might need some skilz or else pay an electronics shop to do it so long as the card is actually working and not broken, is it that terrifying to renew the thermal pads on an out of warranty card in 5 years that is still working? Either way if the card will only continue to work if you fix it up, there's nothing to lose in trying. So you need to know what thermal pads of which thickness to buy - 1 or 2 mm can actually be more tricky than it looks if nobody knows or nobody says in their specification data sheets.
Zotac's warranty terms looked like they intended only to sort DOAs mainly you unbox the card, plug it in, it doesn't work you can send it back the end. If you faff around with it you might be out of luck.
If you decide you want to apply liquid metal instead of thermal grease just don't get it everywhere and realise that spilling liquid metal is deadly to hardware - you decide the risks you want to take. Chances are you don't want to do anything radical to anything expensive, right? So just stick with thermal paste then, don't have a cow, man.
So if the seller is selling the card unused and trying to turn a profit by effectively shorting demand that's one thing if they've opened it up and mined on it and then sold it as new - that's misrepresentation.
Basickly gpus fly off the shelves - you need a reliable reseller and tech companies sprout like mushrooms and disappear overnight, warranties with them.
So it's up to tech firms to limit purchases if they are targeting home users? What can you do? You obviously need to keep informed, people aren't going to say (yet) you can't buy 20 of this product off of us, so who cares is basically what it is besides some people might have a legitimate use for bulk orders so should the industry have some verification of purchaser scheme really will they do that when they are in it for a profit