If its seriously flawed or has issues not disclosed, I will open a case. Ive only had to do this once and it was for a Longchamp bag. I was fully refunded. If you open cases make sure to describe the damage well and take pictures of it too. Best case is, you get your money back, worst case is, you will have to keep the bags and try to find a way to fix them. I don’t think Id be happy keeping a bag if I knew the flaws were purposely omitted by the seller- its disappointing and Id rather know upfront what Id be dealing with! Also for poshmark, just remember you only have 3 days to open a case, so don’t wait too long if thats the route you end up taking
eta: the grace I give depends on how noticeable the flaws are and if the seller is being apologetic about it. Your posh seller seems like she knew about the flaw and is now being defensive because you brought it up-I would open a case. As for your Mercari seller- Im not sure tbh. If the missing rivet will bother you then Id ask for a return but if its not too bad, Id probably just keep it.