[mothered]

Best buy in person se(sealed box method)
Only possible issue is, my old laptop is a 15.6 while the new is a 17.3. Should I still use my old unit for the se?

As long as there's no signs of tampering whatsoever, It's fine.

The objective of the sealed box method, Is to (seemingly) return the Item untouched, thereby It will not give reason to check It.