In the successful fly & buy of several bikes over the years I have always been able to negotiate the use of seller's plate and registration for the return ride home as part of the terms of sale. If the bike was not currently inspected, registered and licensed, I would either require that it be done before the sale, or significantly reduce my offer in expectation of having to arrange temporary travel permits in the state where the sale was to be made, or pass on the purchase altogether.
In the one instance of buying from a long distance dealer, a temporary registration was furnished in my name, details confirmed in advance. I supplied my own insurance in advance, and promptly mailed the seller's license plate and registration back to them upon my return home.
A dated and signed bill of sale with a copy for both buyer and seller, gives some confidence to the seller that should an accident occur on buyer's trip home, the ownership of the bike resides with the buyer as of the date/time of sale, despite the old registration continuing for a few days.
I would advise not short-cutting, that is using a plate or registration that are not specifically and accurately representing the bike you are riding!
Fraudulent documents are sufficient cause for impoundment and fines should an LOE decide to do so. I had a friend left standing in the Nevada desert because he had attempted to ride a bike with license and registration from another bike. Observent LOE who stopped him for speeding, noted VIN # discrepancy, suspected possible theft, and impounded the bike. Friend got mad, said a thing or two he shouldn't have, and officer left him standing in the desert after tow company picked up the impounded bike! Officer wouldn't even give him a ride into town...said that wasn't his problem. Maybe karma on both sides probably will even up the score someday
In the event of an accident involving an unlicensed (un-inspected too?) and un-registered vehicle, it is very likely that the fraud would be exposed. Insurance would probably be voided too. Best not to take short-cuts in my opinion.
Generally following above, I have had no bad experiences in buying long distance. All but one were BMW's, and purchased from folks on one or another of the BMW forums. who I believe are above average in being straight and honorable.