Investigate and do what? It's not illegal to buy GPUs, the sanctions have no power in this space. Who could a law even hurt here, the seller who is a single individual? If they made it illegal to individually export them out of the US the Chinese could just buy them somewhere else.

You can investigate buyers who have anomalous purchase patterns for sanction violations and convict them. DEA commonly looks at narcotic purchases by legal buyers for indications they might be funneling it to illegal market and investigates. NVidia could report "Hey, we are seeing massive purchases from entities we didn't expect so you might want to look into that"