Nvidia and AMD both deal with high demand products (GPUs), and they could apply pressure to eBay to enforce these types of policies, the difficulty lies in eBay’s business model...they profit from listing fees, so they are not going crack down on scalpers willingly.
The idea of restricting these third party listings may work, but I dont think they would be forced to, given how ingrained this practice is on their platform that users and sellers agree to.
In 2014, eBay was hit with a class action lawsuit from users who claimed that the platform facilitated fraudulent and deceptive sales. All it did was cause funds to be refunded to the users involved but I dont think it was anything to write home about. And remember, as long as people are knowingly paying more, and willing, there isn't any fraud.
The FTC has known about these types of exchanges on eBay and has done nothing in the last 30 years.
Maybe a class action lawsuit against eBay would be an answer. That would require a large group of affected consumers to come together and take legal action, and after all they just want a gpu, some will put up their first born for one.
It could create some publicity, and definitely be news worthy, and that could cause eBay to act. That would take a lot of time and resources, but it needs to start somewhere. I just dont thing the "gamer crowd" is interested in putting the effort in it.