In my opinion, if you buy a graded card, you can’t hold the seller responsible if you find any flaw on the card that in your opinion does not match the grade. The seller is not a grader and does not work for the grading company, so can’t be responsible for the grade. The grade has been awarded by the company and is like a certificate about its condition.
So if you are very particular about a certain flaw (in my case I try to get cards without holo scratches), you can ask the seller beforehand if he can see this particular flaw on the card. This can help you with the decision. But even if you find a minor flaw (such as a minor scratch), it is in my opinion not really the sellers responsibility as he is not a professional grader and just tried to do his best to examine the card as a non-professional. As a non-professional, he does not have to have the capacity to assess the card in a “professional” manner.
When you buy a graded card, you essentially buy the grade. If you weren’t able to investigate the condition before the purchase properly and you don’t like the condition after the purchase, its an issue between you and the grading company.
This is definitly frustrating and I have been in the situation myself, but I think that’s the risk that comes with buying graded cards.
But I see it this way: The risk of buying non-graded cards is much higher, as I have to only trust the seller’s description without any standard to uphold to.
PS: Regarding flaws on graded cards. I don’t think there is a flaw that isn’t allowed on any of the grades. In my opinion, you can make a list of flaws (scratches, whitening, creases, nicks etc…) and then you just walk up a scale. The higher you are up the scale, the less pronounced can any of those flaws be. But essentially I think you can get any flaws on higher grades, just less pronounced.
What’s the seller’s responsibility then? In my opinion, the seller has to correctly mention the grade and make sure he mentions any damage to the case (including cracks or fading through light damage). In addition, the seller should be willing to answer as many questions as necessary for you to make a proper decision. The seller should provide detailed photographs of the front and back. If possible high-res scans. In addition, videos showing the holos from various angles help as well. In response to specific questions, the seller should also be able to describe the condition and any possible flaws in his own words and be honest about it, but should not be held responsible for it if he makes a small mistakes (such as in case he overlooks a very minor scratch). I don’t see a need to describe flaws in the item description, but upon request when the buyer asks.
Btw, everything above is just my opinion. I haven’t been in the game of collecting graded cards for long. But I’ve done a lot of research, reading and talking to more experienced collectors.