If the plates are legit, there will be documentation to go with them, as there would be in any other country. If he can't provide them to you, the plates are not legit. You'll need legal papers for a transfer of ownership.

A driver's license is unrelated to whether the bike is legal or not. I don't know whether you need a license to drive a sub-50cc bike, but that's answered here in MCM somewhere.

I do hope someone in the forum can help with specifics.

Sounds dodgy as hell to me.