If he has another plate, he could bring it down, put it on and drive it back. There is some level of risk for each of you, but it is relatively minimal. I've done it a few times in the past. You're relatively protected w/ a notarized bill of sale as long as he adds it to his insurance, he's relatively protected as well.
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Josh (PA) -
'19 X5
'17 991.2 C2 Cab
'11 135i Convertible
'11 328xiT
'09 X3
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