[BoxMann]

I think I found a solution.

I called NY DMV. They can't issue in-transit or temp plates, but if the guy registers the car, puts the plates on the car but doesn't affix the regis sticker to the window, he can return the plates and regis sticker to DMV within 30 days and get a full refund. All he'll have to do is pay for a few days or a few weeks of insurance, which is only a few dollars.

I spoke to the Seller about all this. Looks like he'll do it.

Thank you everyone for your help and advice.