I’m not sure if they’ll take action around this to be honest. I would hope so but this is probably way down their list of priorities at the minute, I’m afraid.
Pedja raised an identical issue a few months back and some of these sellers that you and he have referenced have been the subject of complaints of abuse/scamming/idea theft for years and they are still trading on the site.
I have posted up a conversation by two of them where they have admitted that they are selling games that they do not own and hope to source it for cheaper from other sites like G2A and Kinguin which can lead to keys being cancelled, buyers being given used keys and/or having to wrestle for weeks with Support to have their sale cancelled. In the same conversation they verbally abuse/insult one another over idea theft and copying each others postings so they’re even exploiting each other.
What they do is unhealthy and diminishes the platform but they themselves have said it’s incredibly profitable and holds no risk/investment for them so they won’t stop until Gameflip put some mechanism in place.
I do believe Gameflip is run by a well-intentioned group with aspirations of a legitimate, healthy platform with a strong reputation but developing that kind of reputation/brand requires a foundation of honest, reliable, respectful and mature sellers.